Early life and career
Born in Meaford, Canada West (now Ontario) to a Congregationalist minister, Duff received a Bachelor of Arts in mathematics and metaphysics from the University of Toronto in 1887. After graduation, he taught at Barrie Collegiate Institute while studying for the bar. Duff later took courses at Osgoode Hall Law School and was called to the Ontario Bar in 1893.
Duff practiced as a lawyer in Fergus, Ontario after being called to the bar. In 1895, Duff moved to Victoria, British Columbia and continued his career there. In 1895, he was made a Queen's Counsel. In 1903, he took part, as junior counsel for Canada, in the Alaska Boundary arbitration.