His law practice was interrupted by World War II, where he served as Assistant Deputy Judge Advocate with the Third Canadian Division from 1941 to 1944. After the war he helped found the law firm, Daley, Phinney & Ritchie. He also was a lecturer on insurance law at Dalhousie University.
In 1959, without any previous judicial experience, Ritchie was appointed by the Diefenbaker government to replace Ivan Rand on the Supreme Court of Canada.
He served on the Court until his retirement in 1984. In 1985 he was made a Companion of the Order of Canada. From 1974 to 1988, he was the Chancellor of the University of King's College.