diff --git a/test/test_js_instrument_non_existing_properties.py b/test/test_js_instrument_existing_window_property_with_partially_existing_attributes.py
similarity index 86%
rename from test/test_js_instrument_non_existing_properties.py
rename to test/test_js_instrument_existing_window_property_with_partially_existing_attributes.py
index 75128fe6..0bdf7e8b 100644
--- a/test/test_js_instrument_non_existing_properties.py
+++ b/test/test_js_instrument_existing_window_property_with_partially_existing_attributes.py
@@ -19,13 +19,14 @@ METHOD_CALLS = {
     ('nonExistingMethod1', 'call', '["hello","{\\\"world\\\":true}"]'),
 }
 
+TEST_PAGE = ("instrument_existing_window_property_"
+             "with_partially_existing_attributes.html")
 TOP_URL = (
-    u"%s/js_instrument/instrument_non_existing_properties.html" % (
-        util.BASE_TEST_URL)
+    u"%s/js_instrument/%s" % (util.BASE_TEST_URL, TEST_PAGE)
 )
 
 
-class TestJSInstrumentNonExistingProperties(OpenWPMTest):
+class TestJSInstrumentExistingWindowPropertyWithPartiallyExistingAttributes(OpenWPMTest):
 
     def get_config(self, data_dir=""):
         manager_params, browser_params = self.get_test_config(data_dir)
@@ -56,8 +57,7 @@ class TestJSInstrumentNonExistingProperties(OpenWPMTest):
 
     def test_instrument_object(self):
         """ Ensure instrumentObject logs all property gets, sets, and calls """
-        db = self.visit(
-            '/js_instrument/instrument_non_existing_properties.html')
+        db = self.visit('/js_instrument/%s' % TEST_PAGE)
         rows = db_utils.get_javascript_entries(db, all_columns=True)
 
         # Check calls of non-recursive instrumentation
diff --git a/test/test_js_instrument_mock_window_property.py b/test/test_js_instrument_mock_window_property.py
new file mode 100644
index 00000000..7f0bc247
--- /dev/null
+++ b/test/test_js_instrument_mock_window_property.py
@@ -0,0 +1,64 @@
+from __future__ import absolute_import
+
+import re
+
+from ..automation.utilities import db_utils
+from . import utilities as util
+from .openwpmtest import OpenWPMTest
+
+GETS_AND_SETS = {
+    ("existingProp", "get", "foo"),
+    ("existingProp", "set", "blah1"),
+    ("existingProp", "get", "blah1"),
+    ("nonExistingProp1", "get", "undefined"),
+    ("nonExistingProp1", "set", "blah2"),
+    ("nonExistingProp1", "get", "blah2"),
+}
+
+METHOD_CALLS = {
+    ('nonExistingMethod1', 'call', '["hello","{\\\"world\\\":true}"]'),
+}
+
+TEST_PAGE = "instrument_mock_window_property.html"
+TOP_URL = (
+    u"%s/js_instrument/%s" % (util.BASE_TEST_URL, TEST_PAGE)
+)
+
+
+class TestJSInstrumentMockWindowProperty(OpenWPMTest):
+
+    def get_config(self, data_dir=""):
+        manager_params, browser_params = self.get_test_config(data_dir)
+        browser_params[0]['js_instrument'] = True
+        manager_params['testing'] = True
+        return manager_params, browser_params
+
+    def _check_calls(self, rows, symbol_prefix, doc_url, top_url):
+        """Helper to check method calls and accesses in each frame"""
+        observed_gets_and_sets = set()
+        observed_calls = set()
+        for row in rows:
+            if not row['symbol'].startswith(symbol_prefix):
+                continue
+            symbol = re.sub(symbol_prefix, '', row['symbol'])
+            assert row['document_url'] == doc_url
+            assert row['top_level_url'] == top_url
+            if row['operation'] == 'get' or row['operation'] == 'set':
+                observed_gets_and_sets.add(
+                    (symbol, row['operation'], row['value'])
+                )
+            else:
+                observed_calls.add(
+                    (symbol, row['operation'], row['arguments'])
+                )
+        assert observed_calls == METHOD_CALLS
+        assert observed_gets_and_sets == GETS_AND_SETS
+
+    def test_instrument_object(self):
+        """ Ensure instrumentObject logs all property gets, sets, and calls """
+        db = self.visit('/js_instrument/%s' % TEST_PAGE)
+        rows = db_utils.get_javascript_entries(db, all_columns=True)
+
+        # Check calls of non-recursive instrumentation
+        self._check_calls(rows, 'window.alreadyInstantiatedMockClassInstance', TOP_URL, TOP_URL)
+        self._check_calls(rows, 'window.newMockClassInstance', TOP_URL, TOP_URL)
diff --git a/test/test_js_instrument_non_existing_window_property.py b/test/test_js_instrument_non_existing_window_property.py
index fb72ecec..92132d72 100644
--- a/test/test_js_instrument_non_existing_window_property.py
+++ b/test/test_js_instrument_non_existing_window_property.py
@@ -19,13 +19,13 @@ METHOD_CALLS = {
     ('nonExistingMethod1', 'call', '["hello","{\\\"world\\\":true}"]'),
 }
 
+TEST_PAGE = "instrument_non_existing_window_property.html"
 TOP_URL = (
-    u"%s/js_instrument/instrument_non_existing_object.html" % (
-        util.BASE_TEST_URL)
+    u"%s/js_instrument/%s" % (util.BASE_TEST_URL, TEST_PAGE)
 )
 
 
-class TestJSInstrumentNonExistingObject(OpenWPMTest):
+class TestJSInstrumentNonExistingWindowProperty(OpenWPMTest):
 
     def get_config(self, data_dir=""):
         manager_params, browser_params = self.get_test_config(data_dir)
@@ -56,8 +56,7 @@ class TestJSInstrumentNonExistingObject(OpenWPMTest):
 
     def test_instrument_object(self):
         """ Ensure instrumentObject logs all property gets, sets, and calls """
-        db = self.visit(
-            '/js_instrument/instrument_non_existing_object.html')
+        db = self.visit('/js_instrument/%s' % TEST_PAGE)
         rows = db_utils.get_javascript_entries(db, all_columns=True)
 
         # Check calls of non-recursive instrumentation
diff --git a/test/test_pages/js_instrument/instrument_non_existing_properties.html b/test/test_pages/js_instrument/instrument_existing_window_property_with_partially_existing_attributes.html
similarity index 94%
rename from test/test_pages/js_instrument/instrument_non_existing_properties.html
rename to test/test_pages/js_instrument/instrument_existing_window_property_with_partially_existing_attributes.html
index cfa10db9..ffcd6e61 100644
--- a/test/test_pages/js_instrument/instrument_non_existing_properties.html
+++ b/test/test_pages/js_instrument/instrument_existing_window_property_with_partially_existing_attributes.html
@@ -1,10 +1,10 @@
 <!doctype html>
 <html>
   <head>
-    <title>Test page for JS Instrument's instrumentObject method - specifically testing non-existing properties</title>
+    <title>Test page for JS Instrument's instrumentObject method - specifically instrumenting existing window property with partially existing attributes</title>
   </head>
   <body>
-    <h3>Test page for JS Instrument's instrumentObject method - specifically testing non-existing properties</h3>
+    <h3>Test page for JS Instrument's instrumentObject method - specifically instrumenting existing window property with partially existing attributes</h3>
     <p> This page defines several types of objects and tests the js
     instrument's instrumentObject() method. <br/>
     NOTE: The platform config option `manager_params['testing']` must be set
diff --git a/test/test_pages/js_instrument/instrument_mock_window_property.html b/test/test_pages/js_instrument/instrument_mock_window_property.html
new file mode 100644
index 00000000..e3702976
--- /dev/null
+++ b/test/test_pages/js_instrument/instrument_mock_window_property.html
@@ -0,0 +1,146 @@
+<!doctype html>
+<html>
+  <head>
+    <title>Test page for JS Instrument's instrumentObject method - specifically instrumenting a non-existing window property's attributes via a mock class / object</title>
+  </head>
+  <body>
+    <h3>Test page for JS Instrument's instrumentObject method - specifically instrumenting a non-existing window property's attributes via a mock class / object</h3>
+    <p> This page defines several types of objects and tests the js
+    instrument's instrumentObject() method. <br/>
+    NOTE: The platform config option `manager_params['testing']` must be set
+    to True otherwise the instrumentObject method won't be exposed to this
+    test script.
+    </p>
+    <script type="text/javascript">
+      function interactWithTestObjects() {
+
+        // access the class itself
+        console.log("MockClass",window.MockClass);
+
+        /*
+         * Interact with instrumented instance of the non-instrumented Mock class
+         */
+
+        // get object
+        console.log("alreadyInstantiatedMockClassInstance",window.alreadyInstantiatedMockClassInstance);
+
+        // call object as method
+        try {
+          window.alreadyInstantiatedMockClassInstance('hello', {'world': true});
+        } catch (e) {
+          console.log("call object as method - caught exception: ", e);
+        }
+
+        // get and set nonExistingProp1 (data property 1)
+        try {
+          console.log("nonExistingProp1 before set",window.alreadyInstantiatedMockClassInstance.nonExistingProp1);
+        } catch (e) {
+          console.log("nonExistingProp1 before set - caught exception: ", e);
+        }
+        try {
+          window.alreadyInstantiatedMockClassInstance.nonExistingProp1 = 'blah1';
+        } catch (e) {
+          console.log("nonExistingProp1 set - caught exception: ", e);
+        }
+        try {
+          console.log("nonExistingProp1 after set",window.alreadyInstantiatedMockClassInstance.nonExistingProp1);
+        } catch (e) {
+          console.log("nonExistingProp1 after set - caught exception: ", e);
+        }
+
+        // call non-existing object method
+        try {
+          window.alreadyInstantiatedMockClassInstance.nonExistingMethod1('hello', {'world': true});
+        } catch (e) {
+          console.log("call object method - caught exception: ", e);
+        }
+
+        /*
+         * Interact with instance of the instrumented Mock class
+         */
+
+        // get object
+        console.log("newMockClassInstance",window.newMockClassInstance);
+
+        // call object as method
+        try {
+          window.newMockClassInstance('hello', {'world': true});
+        } catch (e) {
+          console.log("call object as method - caught exception: ", e);
+        }
+
+        // get and set nonExistingProp1 (data property 1)
+        try {
+          console.log("nonExistingProp1 before set",window.newMockClassInstance.nonExistingProp1);
+        } catch (e) {
+          console.log("nonExistingProp1 before set - caught exception: ", e);
+        }
+        try {
+          window.newMockClassInstance.nonExistingProp1 = 'blah1';
+        } catch (e) {
+          console.log("nonExistingProp1 set - caught exception: ", e);
+        }
+        try {
+          console.log("nonExistingProp1 after set",window.newMockClassInstance.nonExistingProp1);
+        } catch (e) {
+          console.log("nonExistingProp1 after set - caught exception: ", e);
+        }
+
+        // call non-existing object method
+        try {
+          window.newMockClassInstance.nonExistingMethod1('hello', {'world': true});
+        } catch (e) {
+          console.log("call object method - caught exception: ", e);
+        }
+
+      }
+
+      // Set up Mock class
+      window.MockClass = function() {};
+
+      // Set up instance of the non-instrumented Mock class
+      window.alreadyInstantiatedMockClassInstance = new MockClass();
+
+      // Instrument mock class non-recursively
+      console.log("Instrumenting window.MockClass!");
+
+      // Instrument the window.MockClass property itself (not its object properties)
+      window.instrumentObject(window, "window", {
+        propertiesToInstrument: ["MockClass"],
+      });
+
+      // Instrument window.MockClass object properties
+      window.instrumentObject(window.MockClass.prototype, "window.MockClass", {
+        propertiesToInstrument: [
+          "existingProp",
+          "nonExistingProp1",
+          "nonExistingMethod1",
+        ],
+      });
+
+      // Instrument the window.alreadyInstantiatedMockClassInstance property itself (not its object properties)
+      window.instrumentObject(window, "window", {
+        propertiesToInstrument: ["alreadyInstantiatedMockClassInstance"],
+      });
+
+      // Instrument window.alreadyInstantiatedMockClassInstance object properties
+      window.instrumentObject(window.alreadyInstantiatedMockClassInstance, "window.alreadyInstantiatedMockClassInstance", {
+        propertiesToInstrument: [
+          "existingProp",
+          "nonExistingProp1",
+          "nonExistingMethod1",
+        ],
+      });
+
+      // Set up instance of the instrumented Mock class
+      window.newMockClassInstance = new MockClass();
+
+      // Instrument the window.newMockClassInstance property itself (not its object properties)
+      window.instrumentObject(window, "window", {
+        propertiesToInstrument: ["newMockClassInstance"],
+      });
+
+      interactWithTestObjects();
+    </script>
+  </body>
+</html>
diff --git a/test/test_pages/js_instrument/instrument_non_existing_window_property.html b/test/test_pages/js_instrument/instrument_non_existing_window_property.html
index 92bdcae8..ac294d4c 100644
--- a/test/test_pages/js_instrument/instrument_non_existing_window_property.html
+++ b/test/test_pages/js_instrument/instrument_non_existing_window_property.html
@@ -1,10 +1,10 @@
 <!doctype html>
 <html>
   <head>
-    <title>Test page for JS Instrument's instrumentObject method - specifically testing non-existing properties</title>
+    <title>Test page for JS Instrument's instrumentObject method - specifically instrumenting non-existing window property</title>
   </head>
   <body>
-    <h3>Test page for JS Instrument's instrumentObject method - specifically testing non-existing properties</h3>
+    <h3>Test page for JS Instrument's instrumentObject method - specifically instrumenting non-existing window property</h3>
     <p> This page defines several types of objects and tests the js
     instrument's instrumentObject() method. <br/>
     NOTE: The platform config option `manager_params['testing']` must be set