He was originally a left back and made his debut on 27 December 1921 against Cardiff City at Ninian Park; Arsenal lost 3–4 in a tightly fought game.
With Arsenal's regular centre forward Henry White out of form and no money available to buy a replacement, Arsenal manager Leslie Knighton switched Turnbull from defence to attack, and the gamble paid off;[2] Turnbull scored 21 league goals that season, including two four-goal hauls against Bolton Wanderers and Blackpool.
An additional goal in the FA Cup against Liverpool brought Turnbull's total for the season to 22, which was then a top-flight record for an Arsenal player.
The arrival of Harry Woods forced Turnbull out of the side altogether and he was sold to Charlton Athletic in November 1924; in all he had scored 28 goals in 66 matches for Arsenal.
[3] After just three months at The Valley, he was sold on to Chelsea, where he rediscovered his form: in less than three seasons he scored 58 goals in 87 first-team games.