Old unreinforced masonry (URM) buildings were often constructed prior to the implementation of modern seismic design standards and are vulnerable to earthquake-induced damage. The high density of old URM buildings in Eastern Canada’s urban centres increases local seismic risk, posing a significant threat to public safety and potential economic loss. Indeed, these vulnerable structures represent a relevant share of buildings across Eastern Canada including schools, fire stations, commercial and residential buildings. Under seismic excitation, failure mechanisms are typically classified into in-plane (IP) or out-of-plane (OOP) modes. Besides OOP mechanisms, IP diagonal shear failures are usually considered most critical because of their brittle nature. Results of IP shear characterization tests are not available in Eastern Canada from in situ tests as they can be difficult and expensive to carry out onsite or with extracted material. Instead, these tests are often completed on samples constructed in the laboratory using modern materials which best represents characteristics of the existing building stock. The research presented in this work discusses the results of diagonal compression tests completed on multiple walls extracted from a downtown Montréal deconstruction site and compares them against analogous tests made on replicated specimens. For the extracted samples, three-wythe clay brick wall samples were carefully removed from a nineteenth-century masonry façade of a typical residential building and transported to McGill University for testing. Replicated walls were constructed in the laboratory using hand-pressed clay bricks and weakened type-O mortar. All walls were tested in diagonal compression according to a modified ASTM E-519 procedure, in addition to characterization tests of units and mortars. Results of this research provide a first look into the diagonal IP behaviour of multi-wythe old URM walls typical of Eastern Canada and relevant details regarding the main differences between response of old vs replicated components. Comparative analysis of these materials not only provides valuable insights into their mechanical properties, but also offers guidance for the retrofitting and preservation of old URM structures.