for the 2012–13 season. He featured regularly for Huddersfield during pre-season receiving his first call up to the Scotland U21s in August 2012. After patiently waiting for his chance, Wallace made his Terriers debut as a substitute in the 1–1 draw against Blackpool at the John Smith's Stadium on 26 December 2012, and made his full debut three days later in their 0–0 draw with Sheffield Wednesday. His first goal for the club came in a 2–2 draw against Peterborough United on 6 April 2013. Wallace capped the season off by winning Young Player of the Year. In the following two seasons, he would make sporadic appearances for the Terriers, mainly due to only being selected when other centre-backs were injured or suspended. Scunthorpe loan On 25 August 2015, with appearances still being limited, Wallace joined Scunthorpe United on loan for a month in an emergency loan deal. After making 12 league appearances, he was recalled by Huddersfield on 1 November, along with team-mate Joe Lolley. Scunthorpe United Wallace signed a three-and-a-half year deal with Scunthorpe on 2 January 2016. Upon signing for the Iron, he featured in every minute of every competitive fixture until 29 August 2017, when his run of 7,560 consecutive playing minutes was broken as he didn't feature in Scunthorpe's EFL Trophy game against Sunderland U21s. He was named as the side's Player of the Season for 2016–17. Millwall In June 2018, Wallace signed a "long-term" deal at Championship side Millwall for a £500,000 fee. He scored the winning goal in two consecutive rounds of the 2018–19 FA Cup as the Lions defeated Everton and AFC Wimbledon to reach the quarter-finals, but also collected two yellow cards and was suspended for the resulting match against Brighton which ended in defeat for Millwall on penalties. Wallace was named as the London side's Player of the Season for 2021–22 season. In July 2022, Wallace signed a new long-term contract. International career Wallace gained his first international call-up for the Scotland U20 side in May 2012 when he was named by head coach Ricky Sbragia in the squad to compete in the 59th ADO Den Haag tournament. He made his debut in a 0–0 draw with Manchester City on 25 May 2012 (the same weekend his club side Huddersfield Town secured promotion to the Championship via the League One play-offs). and went on to feature in all five of