Portus was educated at East Maitland High School (1898-1900), St Paul's College at the University of Sydney (gaining a B.A. in 1906 and a Rhodes Scholarship in 1907) and New College, Oxford (B.A., 1909; B.Litt., 1911; M.A., 1917). He was a foundation scholar at the Hereford theological college, becoming a deacon in 1910. By 1914 he was acting Professor of History and English at the University of Adelaide. Later, for sixteen years from 1918, he was Director of tutorial classes at the University of Sydney. In 1934 he became Professor of History and Political Science at the University of Adelaide, a position he held until his retirement in 1950 (after which he became Professor emeritus).
Portus was a foundation member of the New South Wales group of the Institute of Pacific Relations, and an editor with the Workers' Educational Association. He was also one of the founders of the Australian Institute of Political Science, and a member of the Sydney chapter of the Round Table group (1920-1933). He was part of the South Australian and national talks advisory committees of the Australian Broadcasting Commission, and in his later years was renowned as a radio personality. He published many and varied academic works over his long career.
