diff --git a/traitsui/testing/tester/qt4/implementation/check_list_editor.py b/traitsui/testing/tester/qt4/implementation/check_list_editor.py
index dfaca11ef..f0852f8cc 100644
--- a/traitsui/testing/tester/qt4/implementation/check_list_editor.py
+++ b/traitsui/testing/tester/qt4/implementation/check_list_editor.py
@@ -11,6 +11,7 @@
 
 from traitsui.qt4.check_list_editor import CustomEditor
 from traitsui.testing.tester import command, locator
+from traitsui.testing.tester.editors.layout import column_major_to_row_major
 from traitsui.testing.tester.qt4 import helpers
 
 
@@ -69,12 +70,37 @@ def register(cls, registry):
             interaction_class=command.MouseClick,
             handler=lambda wrapper, _: helpers.mouse_click_qlayout(
                 layout=wrapper.target.target.control.layout(),
-                index=wrapper.target.index,
+                index=convert_index(
+                    wrapper.target.target.control.layout(),
+                    wrapper.target.index
+                ),
                 delay=wrapper.delay,
             )
         )
 
 
+def convert_index(layout, index):
+    """ Helper function to convert an index for a QGridLayout so that the
+    index counts over the grid in the correct direction.
+    The grid is always populated in row major order, but it is done so in
+    such a way that the entries appear in column major order.
+    Qlayouts are indexed in the order they are populated, so to access
+    the correct element we may need to convert a column-major based index
+    into a row-major one.
+
+    Parameters
+    ----------
+    layout : QGridLayout
+        The layout of interest
+    index : int
+        the index of interest
+    """
+    n = layout.count()
+    num_cols = layout.columnCount()
+    num_rows = layout.rowCount()
+    return column_major_to_row_major(index, n, num_rows, num_cols)
+
+
 def register(registry):
     """ Register interactions for the given registry.
 
diff --git a/traitsui/testing/tester/wx/implementation/check_list_editor.py b/traitsui/testing/tester/wx/implementation/check_list_editor.py
index dba4edead..b641de635 100644
--- a/traitsui/testing/tester/wx/implementation/check_list_editor.py
+++ b/traitsui/testing/tester/wx/implementation/check_list_editor.py
@@ -8,9 +8,11 @@
 #
 #  Thanks for using Enthought open source!
 #
+import wx
 
 from traitsui.wx.check_list_editor import CustomEditor
 from traitsui.testing.tester import command, locator
+from traitsui.testing.tester.editors.layout import column_major_to_row_major
 from traitsui.testing.tester.wx import helpers
 
 
@@ -70,11 +72,41 @@ def register(cls, registry):
             handler=lambda wrapper, _:
                 (helpers.mouse_click_checkbox_child_in_panel(
                     control=wrapper.target.target.control,
-                    index=wrapper.target.index,
+                    index=convert_index(
+                        wrapper.target.target,
+                        wrapper.target.index,
+                    ),
                     delay=wrapper.delay))
         )
 
 
+def convert_index(source, index):
+    """ Helper function to convert an index for a GridSizer so that the
+    index counts over the grid in the correct direction.
+    The grid is always populated in row major order, however, the elements
+    are assigned to each entry in the grid so that when displayed they appear
+    in column major order.
+    Sizers are indexed in the order they are populated, so to access
+    the correct element we may need to convert a column-major based index
+    into a row-major one.
+
+    Parameters
+    ----------
+    control : CustomEditor
+        The Custom CheckList Editor of interest. Its control is the wx.Panel
+        containing child objects organized with a wx.GridSizer
+    index : int
+        the index of interest
+    """
+    sizer = source.control.GetSizer()
+    if isinstance(sizer, wx.BoxSizer):
+        return index
+    n = len(source.names)
+    num_cols = sizer.GetCols()
+    num_rows = sizer.GetEffectiveRowsCount()
+    return column_major_to_row_major(index, n, num_rows, num_cols)
+
+
 def register(registry):
     """ Register interactions for the given registry.
 
diff --git a/traitsui/tests/editors/test_check_list_editor.py b/traitsui/tests/editors/test_check_list_editor.py
index 2b65bf0f5..340bc0057 100644
--- a/traitsui/tests/editors/test_check_list_editor.py
+++ b/traitsui/tests/editors/test_check_list_editor.py
@@ -35,6 +35,20 @@ def get_view(style):
     )
 
 
+def get_view_custom_cols(cols):
+    return View(
+        UItem(
+            "value",
+            editor=CheckListEditor(
+                values=["one", "two", "three", "four", "five", "six", "seven"],
+                cols=cols,
+            ),
+            style="custom",
+        ),
+        resizable=True
+    )
+
+
 def get_mapped_view(style):
     return View(
         UItem(
@@ -464,6 +478,20 @@ class StrModel(HasTraits):
 
             self.assertEqual(str_edit.value, "three,one")
 
+    def test_custom_check_list_editor_grid_layout(self):
+        for cols in range(1, 8):
+            list_edit = ListModel()
+            tester = UITester()
+            view = get_view_custom_cols(cols=cols)
+            with tester.create_ui(list_edit, dict(view=view)) as ui:
+                self.assertEqual(list_edit.value, [])
+                check_list = tester.find_by_name(ui, "value")
+                item = check_list.locate(locator.Index(6))
+                item.perform(command.MouseClick())
+                self.assertEqual(list_edit.value, ["seven"])
+                item.perform(command.MouseClick())
+                self.assertEqual(list_edit.value, [])
+
 
 @requires_toolkit([ToolkitName.qt, ToolkitName.wx])
 class TestTextCheckListEditor(unittest.TestCase):