Gallagher started his career at Celtic but never made a first team appearance, however he was involved in their Scottish Youth Cup win in 2010.
Gallagher returned to Livingston after his release from jail, and he played regularly during their consecutive promotions to the top division between 2017 and 2018.
Born in Rutherglen and raised in Blantyre,[3] Gallagher (who also spent time in the setup at Hamilton Academical)[4] signed for Celtic at a young age and played for the under-19s.
He became a regular in the team and played the full 90 minutes in a 2–0 victory over Rangers in the 2010 Scottish Youth Cup final.
[16] He made his debut in a League Cup 0–0 draw against Peterhead (Dundee won on penalties);[17] he also played in the next round of the tournament, a defeat to Queens Park at Hampden.
[23] He scored two goals in two matches as Dundee beat Hamilton Academical 3–0 in late November [24] and Morton 2–1 in early December.
[26] As a result, Gallagher was linked a move away from Dundee, "as Scottish Premiership and English League One clubs have been monitoring his situation".
[32] He scored on his league debut for the Lions in a 2–1 away defeat to Hibernian,[33] and was in the side which won the 2015 Scottish Challenge Cup Final, beating Alloa Athletic.
[34] Gallagher and a co-accused were convicted in June 2015[35] of an assault at a hotel in Blantyre, South Lanarkshire on the evening of a wedding anniversary party in April 2013.
In an attack described by the Sheriff as "brutal", Gallagher struck his victim to the head with a baseball bat or similar implement causing a fractured skull, and was sentenced to three years in prison.
[42] Gallagher was a regular for Livingston as they finished runners-up in the 2017–18 Scottish Championship, and played in all four matches as the club subsequently defeated Dundee United and Partick Thistle in the play-offs to achieve a second consecutive promotion.
[52] On 6 October 2019, Gallagher was called into the Scotland squad for the first time, ahead of the Euro 2020 qualifiers against Russia and San Marino.
[55][56] Gallagher played in the play-off win against Serbia that secured qualification for UEFA Euro 2020, the first time Scotland had qualified for a major tournament since the 1998 World Cup.