After being released by Rangers' youth team, McLean started his senior career at SPL club St Mirren.

He went on loan to Second Division side Arbroath for the second half of the 2009–10 season, before returning to St Mirren and breaking into the first team.

Shortly after this match, Arbroath signed McLean's St Mirren teammate Kyle Faulds on loan.

However, The Herald noted that McLean's "drive and energy" lifted St Mirren and helped them to turn the game around and get a draw.

[26] In the weeks leading up to his award, McLean had been watched by English Championship clubs Burnley and Crystal Palace, with Celtic also reportedly interested in him.

[35] On 16 July 2015, McLean scored his first goal for Aberdeen in a 3–0 win away to HNK Rijeka in the second qualifying round first leg of the Europa League.

[36] McLean scored his first league goal for Aberdeen in a 1–0 win over Dundee United on the opening day of the 2015–16 season.

[38] Having helped the team to finish runners-up in the 2017–18 Scottish Premiership[39] – as in each of his other three campaigns with the club – McLean finally departed Aberdeen after making 158 appearances and scoring 25 goals during his spell at Pittodrie (including the loan back after signing for Norwich).

[41][42][43] Having missed a portion of his first season in English football with an ankle ligament injury,[44] in December 2018 he commented that he was determined to prove he was good enough.

[45] He returned to the Canaries team to help the club gain promotion to the Premier League as winners of the 2018–19 EFL Championship, with his enthusiastic title celebrations (addressing the crowd of supporters as the 'Mayor of Norwich' with a ceremonial hat and bottle of fortified wine) receiving media attention.

[57] He was not selected for the Scotland under-21 team to play in a 2013 European Under-21 Football Championship qualification match against the Netherlands on 14 November 2011.

[27] McLean received his first call-up to the senior Scotland squad in March 2016 for a friendly match against Czech Republic,[58] making his debut by playing 57 minutes.

[62] McLean scored the winning penalties as Scotland beat Israel[63] and Serbia in shootouts to secure qualification for UEFA Euro 2020 via playoffs.

[64] McLean missed the tournament itself due to a knee ligament injury suffered in the last game of the 2020–21 EFL Championship season.

[67] A week later, he appeared as a 67th minute substitute for John McGinn in the opening match of the tournament, where Scotland lost 5–1 to hosts Germany.

[72] Former Scotland international, and McLean's teammate at St Mirren, Gary Teale has compared him to Barry Ferguson.