 During the war he served as a driver in the Royal Field Artillery, and returned to play for Arsenal, who had been elected back to the First Division, once competitive football resumed in 1919.Although by this time 32, he continued to play for Arsenal for another two seasons, and started the 1921-22 season as first-choice left half, before being replaced by the young Tom Whittaker.McKinnon was given a free transfer in the summer of 1922, having played 217 matches for Arsenal, scoring 4 goals.