One of Katharine McLennan's greatest interests was history, specifically the history of the Fortress of Louisbourg in Cape Breton Island. For years she had travelled with her father to London, Paris, Boston, and Ottawa, to find information that could help to tell the story of Louisbourg, including the various sieges as well as the day-to-day life of the people who lived there during the 18th century.

After World War I, Katharine, J.S. and other local supporters, like Archdeacon T.F. Draper from Louisbourg, lobbied to have the federal government preserve the Fortress of Louisbourg site. Their work led to the opening of the Louisbourg Museum in 1936 and the eventual partial reconstruction of the Fortress in the 1960s. Katharine was named "Honorary Curator" of the museum and served in this capacity for over twenty years.

Katharine worked tirelessly to build the Louisbourg Museum. She used her contacts and financial resources to locate and retrieve artifacts from the time of French occupation, including a cannon at Halifax and the Louisbourg bells. As Curator, she was responsible for cataloguing artifacts, mounting displays, and coordinating special events. She wrote articles and gave talks to service groups and the public about the history of Louisbourg. Under her supervision, the Museum gained a rich collection of materials and her knowledge of the Fortress was so detailed that she was able to construct a scale model of the site that remains a valued part of the site’s collection to this day. In A Short History of Louisbourg, Katharine explains her commitment to the museum: "The museum was built in the belief that a visual memorial is a necessary adjunct to a living past, and it is far more illuminating than the most eloquent words."

John Lunn, former Superintendent of Louisbourg, commented on Katharine's role in the restoration of the Fortress of Louisbourg. “Single handedly,” he saide, “she founded and financed the Louisbourg Museum and was the active curator for 20 years. For many more years she was a member of the Committee for Restoration and in her lady-like way she persisted with the authorities until something was done.”

Katharine’s legacy lives on through continued interest and investment in the Fortress of Louisbourg - the largest historic reconstruction in North America and one of Canada's foremost national historic sites.