Mewburn served with the volunteer company (No. 9) of University of Toronto students that was a company of The Queen’s Own Rifles.

On June 1st, he was called out from his final exams to go to Ridgeway to defend Canada against the Fenian invaders. He sustained a head wound. He was left on the field and he died in Fenian custody. The Fenians behaved relatively gallantly everywhere, but in this one case, they treated him rather roughly. He was tied, thrown onto a floor of a cabin and he died on the floor of a cabin.