Gallacher joined Liverpool from Falkirk for £200,000 late in the 2018 January transfer window after he had featured in 17 games for the Scottish Championship club during that season. The left-back had been linked with Barcelona, Manchester United, Rangers and Stoke City since breaking into Falkirk's first-team in 2015 aged just 16. Gallacher was the third player to join the Reds from Falkirk following in the footsteps of goalkeeper Bert Slater (1959) and striker Alan Irvine (1986).

Falkirk's manager, Paul Hartley, was delighted for the promising youngster: “It’s an unbelievable move for Tony. For a player to come from Falkirk and go to Liverpool doesn’t happen a lot. It’s a dream come true, and we can’t stand in the way of that. Hopefully, he goes there and proves how good a talent he is. There’s been a lot of transfer talk surrounding him over the last 18 months since he came into the team, but to go to Liverpool is a little bit surreal for him.”

Gallacher was one of several debutants in the Carabao Cup game against Aston Villa on 17 December 2019 while Liverpool's protagonists were otherwise occupied in Qatar featuring in the Club World Cup. In September 2020 he was loaned to Toronto FC for the remainder of that year's Major League Soccer campaign. On returning to Liverpool in January, Gallacher played eight times in Premier League 2 and left the club on 4 January 2022, signing for St Johnstone on a free transfer. He struggled to make his mark with the club and now plays with Hamilton Academicals in the third tier of Scottish football.