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.
As a collegiate rower, he competed for his institution, and the Magdalen College Coxless four took top honors in the Stewards' Challenge Cup and Visitors' Challenge Cup at Henley Royal Regatta in 1907 and 1908. The Magdalen crew was chosen to represent Britain in Olympic rowing in 1908, with Mackinnon joining Collier Cudmore, John Somers-Smith, and James Angus Gillan.