 Mackinnon was born in Paddington, London, and was educated at Rugby School and Magdalen College, Oxford.He rowed for his college and the Magdalen College Coxless four won the Stewards' Challenge Cup and the Visitors' Challenge Cup at Henley Royal Regatta in 1907 and 1908.The Magdalen crew was chosen to represent Great Britain rowing at the 1908 Summer Olympics, and Mackinnon was in the four with Collier Cudmore, John Somers-Smith and James Angus Gillan.