diff --git a/lms/djangoapps/django_comment_client/tests/utils.py b/lms/djangoapps/django_comment_client/tests/utils.py
index 399c8c10e1..e9c0ca966b 100644
--- a/lms/djangoapps/django_comment_client/tests/utils.py
+++ b/lms/djangoapps/django_comment_client/tests/utils.py
@@ -34,19 +34,19 @@ class CohortedTestCase(SharedModuleStoreTestCase):
     def setUp(self):
         super(CohortedTestCase, self).setUp()
 
-        self.student_cohort = CohortFactory.create(
-            name="student_cohort",
-            course_id=self.course.id
-        )
-        self.moderator_cohort = CohortFactory.create(
-            name="moderator_cohort",
-            course_id=self.course.id
-        )
         seed_permissions_roles(self.course.id)
         self.student = UserFactory.create()
         self.moderator = UserFactory.create()
         CourseEnrollmentFactory(user=self.student, course_id=self.course.id)
         CourseEnrollmentFactory(user=self.moderator, course_id=self.course.id)
         self.moderator.roles.add(Role.objects.get(name="Moderator", course_id=self.course.id))
-        self.student_cohort.users.add(self.student)
-        self.moderator_cohort.users.add(self.moderator)
+        self.student_cohort = CohortFactory.create(
+            name="student_cohort",
+            course_id=self.course.id,
+            users=[self.student]
+        )
+        self.moderator_cohort = CohortFactory.create(
+            name="moderator_cohort",
+            course_id=self.course.id,
+            users=[self.moderator]
+        )
diff --git a/lms/djangoapps/instructor_analytics/tests/test_basic.py b/lms/djangoapps/instructor_analytics/tests/test_basic.py
index daa3b45e7a..efd98560d0 100644
--- a/lms/djangoapps/instructor_analytics/tests/test_basic.py
+++ b/lms/djangoapps/instructor_analytics/tests/test_basic.py
@@ -133,8 +133,8 @@ class TestAnalyticsBasic(ModuleStoreTestCase):
         course = CourseFactory.create(org="test", course="course1", display_name="run1")
         course.cohort_config = {'cohorted': True, 'auto_cohort': True, 'auto_cohort_groups': ['cohort']}
         self.store.update_item(course, self.instructor.id)
-        cohort = CohortFactory.create(name='cohort', course_id=course.id)
         cohorted_students = [UserFactory.create() for _ in xrange(10)]
+        cohort = CohortFactory.create(name='cohort', course_id=course.id, users=cohorted_students)
         cohorted_usernames = [student.username for student in cohorted_students]
         non_cohorted_student = UserFactory.create()
         for student in cohorted_students:
diff --git a/lms/djangoapps/instructor_task/tasks_helper.py b/lms/djangoapps/instructor_task/tasks_helper.py
index f277f28e2c..8808db2a5a 100644
--- a/lms/djangoapps/instructor_task/tasks_helper.py
+++ b/lms/djangoapps/instructor_task/tasks_helper.py
@@ -1505,8 +1505,7 @@ def cohort_students_and_upload(_xmodule_instance_args, _entry_id, course_id, tas
                 continue
 
             try:
-                with outer_atomic():
-                    add_user_to_cohort(cohorts_status[cohort_name]['cohort'], username_or_email)
+                add_user_to_cohort(cohorts_status[cohort_name]['cohort'], username_or_email)
                 cohorts_status[cohort_name]['Students Added'] += 1
                 task_progress.succeeded += 1
             except User.DoesNotExist:
diff --git a/lms/djangoapps/mobile_api/video_outlines/tests.py b/lms/djangoapps/mobile_api/video_outlines/tests.py
index 3e8f0a2f14..ef4d160653 100644
--- a/lms/djangoapps/mobile_api/video_outlines/tests.py
+++ b/lms/djangoapps/mobile_api/video_outlines/tests.py
@@ -17,6 +17,7 @@ from xmodule.partitions.partitions import Group, UserPartition
 
 from openedx.core.djangoapps.course_groups.tests.helpers import CohortFactory
 from openedx.core.djangoapps.course_groups.models import CourseUserGroupPartitionGroup
+from openedx.core.djangoapps.course_groups.cohorts import add_user_to_cohort, remove_user_from_cohort
 
 from ..testutils import MobileAPITestCase, MobileAuthTestMixin, MobileCourseAccessTestMixin
 
@@ -744,7 +745,7 @@ class TestVideoSummaryList(
 
         for cohort_index in range(len(cohorts)):
             # add user to this cohort
-            cohorts[cohort_index].users.add(self.user)
+            add_user_to_cohort(cohorts[cohort_index], self.user.username)
 
             # should only see video for this cohort
             video_outline = self.api_response().data
@@ -755,7 +756,7 @@ class TestVideoSummaryList(
             )
 
             # remove user from this cohort
-            cohorts[cohort_index].users.remove(self.user)
+            remove_user_from_cohort(cohorts[cohort_index], self.user.username)
 
         # un-cohorted user should see no videos
         video_outline = self.api_response().data
diff --git a/lms/djangoapps/notifier_api/tests.py b/lms/djangoapps/notifier_api/tests.py
index 4d040d3564..4b173c77f5 100644
--- a/lms/djangoapps/notifier_api/tests.py
+++ b/lms/djangoapps/notifier_api/tests.py
@@ -5,7 +5,7 @@ from django.conf import settings
 from django.test.client import RequestFactory
 from django.test.utils import override_settings
 
-from openedx.core.djangoapps.course_groups.models import CourseUserGroup
+from openedx.core.djangoapps.course_groups.tests.helpers import CohortFactory
 from django_comment_common.models import Role, Permission
 from lang_pref import LANGUAGE_KEY
 from notification_prefs import NOTIFICATION_PREF_KEY
@@ -46,12 +46,11 @@ class NotifierUsersViewSetTest(UrlResetMixin, ModuleStoreTestCase):
         self.courses.append(course)
         CourseEnrollmentFactory(user=self.user, course_id=course.id)
         if is_user_cohorted:
-            cohort = CourseUserGroup.objects.create(
+            cohort = CohortFactory.create(
                 name="Test Cohort",
                 course_id=course.id,
-                group_type=CourseUserGroup.COHORT
+                users=[self.user]
             )
-            cohort.users.add(self.user)
             self.cohorts.append(cohort)
         if is_moderator:
             moderator_perm, _ = Permission.objects.get_or_create(name="see_all_cohorts")
diff --git a/openedx/core/djangoapps/course_groups/cohorts.py b/openedx/core/djangoapps/course_groups/cohorts.py
index 794c41b65f..cf4827cabb 100644
--- a/openedx/core/djangoapps/course_groups/cohorts.py
+++ b/openedx/core/djangoapps/course_groups/cohorts.py
@@ -181,13 +181,12 @@ def get_cohort(user, course_key, assign=True, use_cached=False):
 
     # If course is cohorted, check if the user already has a cohort.
     try:
-        cohort = CourseUserGroup.objects.get(
+        membership = CohortMembership.objects.get(
             course_id=course_key,
-            group_type=CourseUserGroup.COHORT,
-            users__id=user.id,
+            user_id=user.id,
         )
-        return request_cache.data.setdefault(cache_key, cohort)
-    except CourseUserGroup.DoesNotExist:
+        return request_cache.data.setdefault(cache_key, membership.course_user_group)
+    except CohortMembership.DoesNotExist:
         # Didn't find the group. If we do not want to assign, return here.
         if not assign:
             # Do not cache the cohort here, because in the next call assign
@@ -195,6 +194,17 @@ def get_cohort(user, course_key, assign=True, use_cached=False):
             return None
 
     # Otherwise assign the user a cohort.
+    membership = CohortMembership.objects.create(
+        user=user,
+        course_user_group=_get_default_cohort(course_key)
+    )
+    return request_cache.data.setdefault(cache_key, membership.course_user_group)
+
+
+def _get_default_cohort(course_key):
+    """
+    Helper method to get a default cohort for assignment in get_cohort
+    """
     course = courses.get_course(course_key)
     cohorts = get_course_cohorts(course, assignment_type=CourseCohort.RANDOM)
     if cohorts:
@@ -205,11 +215,7 @@ def get_cohort(user, course_key, assign=True, use_cached=False):
             course_id=course_key,
             assignment_type=CourseCohort.RANDOM
         ).course_user_group
-
-    membership = CohortMembership(course_user_group=cohort, user=user)
-    membership.save()
-
-    return request_cache.data.setdefault(cache_key, cohort)
+    return cohort
 
 
 def migrate_cohort_settings(course):
@@ -332,6 +338,27 @@ def is_cohort_exists(course_key, name):
     return CourseUserGroup.objects.filter(course_id=course_key, group_type=CourseUserGroup.COHORT, name=name).exists()
 
 
+def remove_user_from_cohort(cohort, username_or_email):
+    """
+    Look up the given user, and if successful, remove them from the specified cohort.
+
+    Arguments:
+        cohort: CourseUserGroup
+        username_or_email: string.  Treated as email if has '@'
+
+    Raises:
+        User.DoesNotExist if can't find user.
+        ValueError if user not already present in this cohort.
+    """
+    user = get_user_by_username_or_email(username_or_email)
+
+    try:
+        membership = CohortMembership.objects.get(course_user_group=cohort, user=user)
+        membership.delete()
+    except CohortMembership.DoesNotExist:
+        raise ValueError("User {} was not present in cohort {}".format(username_or_email, cohort))
+
+
 def add_user_to_cohort(cohort, username_or_email):
     """
     Look up the given user, and if successful, add them to the specified cohort.
@@ -350,7 +377,7 @@ def add_user_to_cohort(cohort, username_or_email):
     user = get_user_by_username_or_email(username_or_email)
 
     membership = CohortMembership(course_user_group=cohort, user=user)
-    membership.save()
+    membership.save()  # This will handle both cases, creation and updating, of a CohortMembership for this user.
 
     tracker.emit(
         "edx.cohort.user_add_requested",
diff --git a/openedx/core/djangoapps/course_groups/models.py b/openedx/core/djangoapps/course_groups/models.py
index 3fa34f0f47..7d1c15f303 100644
--- a/openedx/core/djangoapps/course_groups/models.py
+++ b/openedx/core/djangoapps/course_groups/models.py
@@ -7,7 +7,10 @@ import logging
 
 from django.contrib.auth.models import User
 from django.db import models, transaction, IntegrityError
+from util.db import outer_atomic
 from django.core.exceptions import ValidationError
+from django.db.models.signals import pre_delete
+from django.dispatch import receiver
 from xmodule_django.models import CourseKeyField
 
 log = logging.getLogger(__name__)
@@ -85,55 +88,63 @@ class CohortMembership(models.Model):
             raise ValidationError("Non-matching course_ids provided")
 
     def save(self, *args, **kwargs):
-        # Avoid infinite recursion if creating from get_or_create() call below.
-        if 'force_insert' in kwargs and kwargs['force_insert'] is True:
-            super(CohortMembership, self).save(*args, **kwargs)
-            return
-
         self.full_clean(validate_unique=False)
 
-        # This loop has been created to allow for optimistic locking, and retrial in case of losing a race condition.
-        # The limit is 2, since select_for_update ensures atomic updates. Creation is the only possible race condition.
-        max_retries = 2
-        success = False
-        for __ in range(max_retries):
-
+        # Avoid infinite recursion if creating from get_or_create() call below.
+        # This block also allows middleware to use CohortMembership.get_or_create without worrying about outer_atomic
+        if 'force_insert' in kwargs and kwargs['force_insert'] is True:
             with transaction.atomic():
-
-                try:
-                    with transaction.atomic():
-                        saved_membership, created = CohortMembership.objects.select_for_update().get_or_create(
-                            user__id=self.user.id,
-                            course_id=self.course_id,
-                            defaults={
-                                'course_user_group': self.course_user_group,
-                                'user': self.user
-                            }
-                        )
-                except IntegrityError:  # This can happen if simultaneous requests try to create a membership
-                    continue
-
-                if not created:
-                    if saved_membership.course_user_group == self.course_user_group:
-                        raise ValueError("User {user_name} already present in cohort {cohort_name}".format(
-                            user_name=self.user.username,
-                            cohort_name=self.course_user_group.name
-                        ))
-                    self.previous_cohort = saved_membership.course_user_group
-                    self.previous_cohort_name = saved_membership.course_user_group.name
-                    self.previous_cohort_id = saved_membership.course_user_group.id
-                    self.previous_cohort.users.remove(self.user)
-
-                saved_membership.course_user_group = self.course_user_group
                 self.course_user_group.users.add(self.user)
+                self.course_user_group.save()
+                super(CohortMembership, self).save(*args, **kwargs)
+            return
 
-                super(CohortMembership, saved_membership).save(update_fields=['course_user_group'])
+        # This block will transactionally commit updates to CohortMembership and underlying course_user_groups.
+        # Note the use of outer_atomic, which guarantees that operations are committed to the database on block exit.
+        # If called from a view method, that method must be marked with @transaction.non_atomic_requests.
+        with outer_atomic(read_committed=True):
 
-            success = True
-            break
+            saved_membership, created = CohortMembership.objects.select_for_update().get_or_create(
+                user__id=self.user.id,
+                course_id=self.course_id,
+                defaults={
+                    'course_user_group': self.course_user_group,
+                    'user': self.user
+                }
+            )
 
-        if not success:
-            raise IntegrityError("Unable to save membership after {} tries, aborting.".format(max_retries))
+            # If the membership was newly created, all the validation and course_user_group logic was settled
+            # with a call to self.save(force_insert=True), which gets handled above.
+            if created:
+                return
+
+            if saved_membership.course_user_group == self.course_user_group:
+                raise ValueError("User {user_name} already present in cohort {cohort_name}".format(
+                    user_name=self.user.username,
+                    cohort_name=self.course_user_group.name
+                ))
+            self.previous_cohort = saved_membership.course_user_group
+            self.previous_cohort_name = saved_membership.course_user_group.name
+            self.previous_cohort_id = saved_membership.course_user_group.id
+            self.previous_cohort.users.remove(self.user)
+            self.previous_cohort.save()
+
+            saved_membership.course_user_group = self.course_user_group
+            self.course_user_group.users.add(self.user)
+            self.course_user_group.save()
+
+            super(CohortMembership, saved_membership).save(update_fields=['course_user_group'])
+
+
+# Needs to exist outside class definition in order to use 'sender=CohortMembership'
+@receiver(pre_delete, sender=CohortMembership)
+def remove_user_from_cohort(sender, instance, **kwargs):  # pylint: disable=unused-argument
+    """
+    Ensures that when a CohortMemebrship is deleted, the underlying CourseUserGroup
+    has its users list updated to reflect the change as well.
+    """
+    instance.course_user_group.users.remove(instance.user)
+    instance.course_user_group.save()
 
 
 class CourseUserGroupPartitionGroup(models.Model):
diff --git a/openedx/core/djangoapps/course_groups/tests/helpers.py b/openedx/core/djangoapps/course_groups/tests/helpers.py
index d5c3cf38a1..f5d09dfad2 100644
--- a/openedx/core/djangoapps/course_groups/tests/helpers.py
+++ b/openedx/core/djangoapps/course_groups/tests/helpers.py
@@ -32,6 +32,11 @@ class CohortFactory(DjangoModelFactory):
         """
         if extracted:
             self.users.add(*extracted)
+            for user in self.users.all():
+                CohortMembership.objects.create(
+                    user=user,
+                    course_user_group=self,
+                )
 
 
 class CourseCohortFactory(DjangoModelFactory):
@@ -41,18 +46,6 @@ class CourseCohortFactory(DjangoModelFactory):
     class Meta(object):
         model = CourseCohort
 
-    @post_generation
-    def memberships(self, create, extracted, **kwargs):  # pylint: disable=unused-argument
-        """
-        Returns the memberships linking users to this cohort.
-        """
-        for user in self.course_user_group.users.all():  # pylint: disable=E1101
-            membership = CohortMembership(user=user, course_user_group=self.course_user_group)
-            membership.save()
-
-    course_user_group = factory.SubFactory(CohortFactory)
-    assignment_type = 'manual'
-
 
 class CourseCohortSettingsFactory(DjangoModelFactory):
     """
diff --git a/openedx/core/djangoapps/course_groups/tests/test_cohorts.py b/openedx/core/djangoapps/course_groups/tests/test_cohorts.py
index d72c53a4c0..3771563702 100644
--- a/openedx/core/djangoapps/course_groups/tests/test_cohorts.py
+++ b/openedx/core/djangoapps/course_groups/tests/test_cohorts.py
@@ -179,8 +179,7 @@ class TestCohorts(ModuleStoreTestCase):
         self.assertIsNone(cohorts.get_cohort_id(user, course.id))
 
         config_course_cohorts(course, is_cohorted=True)
-        cohort = CohortFactory(course_id=course.id, name="TestCohort")
-        cohort.users.add(user)
+        cohort = CohortFactory(course_id=course.id, name="TestCohort", users=[user])
         self.assertEqual(cohorts.get_cohort_id(user, course.id), cohort.id)
 
         self.assertRaises(
@@ -237,8 +236,7 @@ class TestCohorts(ModuleStoreTestCase):
 
         self.assertIsNone(cohorts.get_cohort(user, course.id), "No cohort created yet")
 
-        cohort = CohortFactory(course_id=course.id, name="TestCohort")
-        cohort.users.add(user)
+        cohort = CohortFactory(course_id=course.id, name="TestCohort", users=[user])
 
         self.assertIsNone(
             cohorts.get_cohort(user, course.id),
@@ -261,8 +259,8 @@ class TestCohorts(ModuleStoreTestCase):
         )
 
     @ddt.data(
-        (True, 2),
-        (False, 6),
+        (True, 3),
+        (False, 9),
     )
     @ddt.unpack
     def test_get_cohort_sql_queries(self, use_cached, num_sql_queries):
@@ -271,10 +269,8 @@ class TestCohorts(ModuleStoreTestCase):
         """
         course = modulestore().get_course(self.toy_course_key)
         config_course_cohorts(course, is_cohorted=True)
-        cohort = CohortFactory(course_id=course.id, name="TestCohort")
-
         user = UserFactory(username="test", email="a@b.com")
-        cohort.users.add(user)
+        CohortFactory.create(course_id=course.id, name="TestCohort", users=[user])
 
         with self.assertNumQueries(num_sql_queries):
             for __ in range(3):
@@ -314,10 +310,7 @@ class TestCohorts(ModuleStoreTestCase):
         user1 = UserFactory(username="test", email="a@b.com")
         user2 = UserFactory(username="test2", email="a2@b.com")
 
-        cohort = CohortFactory(course_id=course.id, name="TestCohort")
-
-        # user1 manually added to a cohort
-        cohort.users.add(user1)
+        cohort = CohortFactory(course_id=course.id, name="TestCohort", users=[user1])
 
         # Add an auto_cohort_group to the course...
         config_course_cohorts(
diff --git a/openedx/core/djangoapps/course_groups/tests/test_partition_scheme.py b/openedx/core/djangoapps/course_groups/tests/test_partition_scheme.py
index 093eca653f..f53708e314 100644
--- a/openedx/core/djangoapps/course_groups/tests/test_partition_scheme.py
+++ b/openedx/core/djangoapps/course_groups/tests/test_partition_scheme.py
@@ -21,7 +21,7 @@ from openedx.core.djangoapps.user_api.partition_schemes import RandomUserPartiti
 from ..partition_scheme import CohortPartitionScheme, get_cohorted_user_partition
 from ..models import CourseUserGroupPartitionGroup
 from ..views import link_cohort_to_partition_group, unlink_cohort_partition_group
-from ..cohorts import add_user_to_cohort, get_course_cohorts
+from ..cohorts import add_user_to_cohort, remove_user_from_cohort, get_course_cohorts
 from .helpers import CohortFactory, config_course_cohorts
 
 
@@ -100,7 +100,7 @@ class TestCohortPartitionScheme(ModuleStoreTestCase):
         self.assert_student_in_group(self.groups[1])
 
         # move the student out of the cohort
-        second_cohort.users.remove(self.student)
+        remove_user_from_cohort(second_cohort, self.student.username)
         self.assert_student_in_group(None)
 
     def test_cohort_partition_group_assignment(self):
diff --git a/openedx/core/djangoapps/course_groups/views.py b/openedx/core/djangoapps/course_groups/views.py
index 373991b57e..3e8e0f5ae9 100644
--- a/openedx/core/djangoapps/course_groups/views.py
+++ b/openedx/core/djangoapps/course_groups/views.py
@@ -10,6 +10,7 @@ from django.core.urlresolvers import reverse
 from django.http import Http404, HttpResponseBadRequest
 from django.views.decorators.http import require_http_methods
 from util.json_request import expect_json, JsonResponse
+from django.db import transaction
 from django.contrib.auth.decorators import login_required
 from django.utils.translation import ugettext
 
@@ -23,7 +24,7 @@ from edxmako.shortcuts import render_to_response
 
 from . import cohorts
 from lms.djangoapps.django_comment_client.utils import get_discussion_category_map, get_discussion_categories_ids
-from .models import CourseUserGroup, CourseUserGroupPartitionGroup
+from .models import CourseUserGroup, CourseUserGroupPartitionGroup, CohortMembership
 
 log = logging.getLogger(__name__)
 
@@ -299,6 +300,7 @@ def users_in_cohort(request, course_key_string, cohort_id):
                                'users': user_info})
 
 
+@transaction.non_atomic_requests
 @ensure_csrf_cookie
 @require_POST
 def add_users_to_cohort(request, course_key_string, cohort_id):
@@ -384,16 +386,22 @@ def remove_user_from_cohort(request, course_key_string, cohort_id):
         return json_http_response({'success': False,
                                    'msg': 'No username specified'})
 
-    cohort = cohorts.get_cohort_by_id(course_key, cohort_id)
     try:
         user = User.objects.get(username=username)
-        cohort.users.remove(user)
-        return json_http_response({'success': True})
     except User.DoesNotExist:
         log.debug('no user')
         return json_http_response({'success': False,
                                    'msg': "No user '{0}'".format(username)})
 
+    try:
+        membership = CohortMembership.objects.get(user=user, course_id=course_key)
+        membership.delete()
+
+    except CohortMembership.DoesNotExist:
+        pass
+
+    return json_http_response({'success': True})
+
 
 def debug_cohort_mgmt(request, course_key_string):
     """