Born at Marida Yallock near Boorcan in Victoria to grazier David Mackinnon and Jane Kinross, both Scottish-born, he was educated at Geelong Grammar School, Melbourne University and New College, Oxford, graduating with a Bachelor of Arts.
In 1883 he was called to the English Bar, returning to Victoria to repeat the achievement in 1884.
[1] In 1900 Mackinnon was elected to the Victorian Legislative Assembly as the Liberal member for Prahran, serving as party leader from 1904 to 1907.
[1] After the failure of the first plebiscite on conscription during World War I, Mackinnon was appointed Commonwealth Director-General of Recruiting on 29 November 1916.
[1] Mackinnon's son Dan was a member of the Australian House of Representatives, and his grandson Donald James was also a Victorian state MP.