William Sellar (1910–1968) was born in Montreal on 11 December 1910 to George Sellar and Margaret Campbell MacIntyre. Sellar attended West Hill High School, and then McGill University where he graduated Bachelor of Arts in 1932 and Bachelor of Laws in 1935. During his university years he also worked as a part-time sports reporter for the Montreal Daily Star. Upon graduation he worked for the Canadian Pacific Railway where he was assitant to vice-president Eric A. Leslie. During World War II Sellar served as an officer in the Royal Canadian Air Force in Britain and Vancouver.

After the war Sellar settled in Calgary and was admitted to the Alberta Bar. At this time he joined the firm Macleod Riley McDermid and Dixon. In 1948 he started his own firm, which became Sellar Densmore and Eamon. Sellar was a member of the Glencoe Club, Ranchmen's Club, and Alberta Provincial Progressive Conservative Association.

In 1938 Sellar married Irene Margaret Johnson (1910–1999) at her parents' home in Calgary. The couple had met a McGill, where she graduated Bachelor of Science in 1934. They had two children, Marilyn and Donald. Sellar died on 19 May 1968 at age 57.