On 12 July 2017, Mackay-Steven signed a two-year contract with Aberdeen for an undisclosed fee.  He made his debut the following day in a UEFA Europa League second qualifying round first leg at home to NK Široki Brijeg, starting in a 1–1 draw. In the second leg in Bosnia a week after, he scored to wrap up a 2–0 win and advancement.

Mackay-Steven scored his first league goal for Aberdeen on 14 October, the only one of a victory at Hibernian.  On 16 December, against the same opponents, he scored his first senior hat-trick in a 4–1 win.

Mackay-Steven made his only international appearance for Scotland on 15 November 2013, replacing Craig Conway for the last six minutes of a goalless friendly draw against the United States at Hampden Park.