McKellar Stewart was a son of Alexander Stewart and his wife Lillias Stewart, née McKellar. Stewart was employed as trainee teacher at Warrnambool State School, then taught at Charlton then Benalla. He entered Ormond College, University of Melbourne, where he graduated in philosophy in 1906 with first class honours, winning the Hastie Scholarship in Logic and Philosophy and from 1907 to 1909 was employed delivering lectures in Logic and Philosophy to Ormond College students, meanwhile studying theology and was ordained a minister of the Presbyterian Church. In 1909 he married Margaret Grace Stuart Bothroyd MA. In 1911 he completed his D.Phil. at the University of Edinburgh, his thesis being on the work of Henri Bergson, later published as Critical Exposition of Bergson's Philosophy, to some acclaim. He undertook further studies at the University of Marburg. He returned to the University of Melbourne in 1912 as a lecturer in philosophy, and entered wholeheartedly into the intellectual and social life of the university. He was heavily involved with the University Union, the controversial Public Questions Society, and the Student Christian Movement and served as chairman of each. He took part in University Extension activities, and for several years served as its secretary. in 1920 Stewart was appointed associate professor of philosophy, a post created for him, then in 1922 was offered the Hughes Professorship in Philosophy on the recommendation of Professor Mitchell, the outgoing incumbent, and was appointed the following year. He was appointed deputy vice-chancellor in 1943 and vice-chancellor in 1945, maintaining his academic duties, the last person to combine the two roles. He retired as vice-chancellor in 1948, but remained professor of philosophy until his retirement in 1950, and was made professor emeritus by the University Council. Other positions he held at Adelaide University at various times included chairman of the University Public Examinations Board and chairman of the university joint committee for tutorial classes. He was foundation chairman of the Students' Union.
Other activities
Stewart was member of the State Advisory Council for Education.
He was appointed to the Government Commission which enquired into education in SA in 1933.
Stewart married Margaret Grace "Madge" Bothroyd on 3 November 1909. Margaret was a founder of the Blackwood branch of the Women's Non-Party Association, which merged into the SA League of Women Voters.
eldest son John S. M. Stewart MB BS married Joyce Spencer Job on 14 November 1942. Joyce was daughter of Roy Job.
Margaret McKellar Stewart MA married Maurice Meredith Stericker Finnis MA on 11 August 1939. Margaret, educated at Walford School and Adelaide University, was president, League of Women Voters. Maurice was the only son of Rev. Horace Percy Finnis, organist at St Peter's Cathedral 1936–1955. Their children were
