*Club domestic league appearances and goals, correct as of 22:30, 31 March 2023 (UTC) ‡ National team caps and goals, correct as of 20:54, 5 October 2019 (UTC)
Mallan scored his first goal on his fifth appearance, the team's second in a 3–1 away win against Dundee at Dens Park.[5][6]BBC Sport described the goal as a "contender for goal of the season".[7] Mallan was subsequently presented with the PFA Scotland 'Goal of the Season' award for that effort.[8] Mallan also won the SPFL Young Player of the Month award for December 2014.[9] Eight days later he scored his second goal, the 87th-minute winner in a 2–1 victory at Ross County.[10] Later in January, Mallan signed a three-year contract extension with St Mirren.[11] He scored a late winning goal against Motherwell on 16 May, but St Mirren had already been relegated to the Championship.[12]
In October 2016 at Easter Road, he scored a "magnificent" free kick from 30 yards to help St Mirren eliminate Hibernian from the Challenge Cup.[15] Mallan left St Mirren after the 2016–17 season, having made over 100 appearances for the club.[16]
Barnsley
On 18 May 2017 Mallan completed a transfer to EFL Championship side Barnsley for an undisclosed fee.[17] He made his competitive debut against Preston North End on Boxing Day in a 0–0 draw. Mallan made only nine appearances for Barnsley during the 2017–18 season, as they were relegated to EFL League One.[16]
Hibernian
Mallan returned to Scottish football in July 2018, moving to Premiership club Hibernian for an undisclosed fee on a 4-year contract.[16] He made his debut for the club on 12 July in a 2018–19 UEFA Europa League qualification tie at home to Faroese side NSÍ Runavík, scoring twice in a 6–1 Hibs win.[18] Mallan had a productive first season with Hibs, scoring 13 goals in 48 appearances. After 3 goals in 26 appearances during the first part of the 2019–20 season, he suffered a knee injury in late December.[19]
Yeni Malatyaspor
Mallan was loaned to Turkish Super Lig club Yeni Malatyaspor for the rest of the season, with an option to buy at the end of the season, on 1 February 2021.[20] In July, Mallan moved on a permanent basis to Malatyaspor, signing a two-year contract with the club.[21]
Salford City
On 30 June 2022, Mallan returned to England to join League Two club Salford City on a two-year deal.[22] On 21 May 2024, the club announced he would be leaving in the summer when his contract expired.[23]