Born in Thurso, Caithness, Mackay-Steven began his youth career with Ross County before moving to English Premier League side Liverpool in 2007.

[4] He made his debut a week later in the fourth round of the Scottish Cup, starting against Greenock Morton at Cappielow Park and scoring the opening goal of a 2–2 draw from 25 yards out.

[7] He made his debut eight days later as an 80th-minute substitute for Johnny Russell in a UEFA Europa League match against Śląsk Wrocław, which his club won 3–2 but were nonetheless eliminated on the away goals rule.

[8] On 24 July, he made his league debut for the club on the opening day of the season, coming on for Barry Douglas for the last 30 minutes of a 1–1 draw against Kilmarnock at Tannadice Park.

[11] Blackburn Rovers scouts were reported to have been watching Mackay-Steven on 24 August 2013, in Dundee United's 4–0 victory over St Johnstone, a game in which he scored.

[15] On 16 January 2015, Mackay-Steven signed a pre-contract agreement to join Celtic at the end of the season on the expiry of his Dundee United contract.

[16] On the transfer deadline day, 2 February, Celtic made an offer of £250,000 and the move was completed immediately, with his Dundee United teammate Stuart Armstrong joining in a separate deal at the same time.

[22] 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.

[26] In the 2018 Scottish League Cup final against former club Celtic on 2 December 2018, Mackay-Steven was stretchered from the field after being knocked unconscious by a clash of heads with Dedryck Boyata shortly before the end of the first half (Aberdeen lost 1–0).

[37] Mackay-Steven made his first 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.