[5] Foster made his debut for Aberdeen aged 17 on 10 May 2003 in a Scottish Premier League match against Partick Thistle, coming on as a substitute for Darren Mackie.
On 20 September 2007, he made his European debut in the first leg of Aberdeen's UEFA Cup first round tie against Dnipro, coming on as a 72nd-minute substitute for Jackie McNamara as the game ended 0–0.
[8] Foster started at left back in the return match in Ukraine on 4 October and provided the assist for Darren Mackie's opening goal, sending in the cross for the striker to head home from close range.
On 20 December, Foster scored the first European goal of his career in Aberdeen's 4–0 UEFA Cup group stage win over Copenhagen.
[17] He was given the number 12 shirt[18] and made his Rangers debut on 11 September, coming on as a substitute for Kirk Broadfoot in a 2–1 victory over Hamilton Accies.
However he was cup-tied for their victorious 2010–11 Scottish League Cup campaign after being an unused substitute for Aberdeen against Alloa Athletic earlier in the season.
[28] After being released by Rangers,[29] Foster signed a two-year contract with Scottish Premiership club Ross County in June 2015.
[33] In April 2017, Foster brought St Johnstone in to disrepute after becoming involved in an on-pitch brawl with teammate Danny Swanson.
[38] On 25 May 2020, Foster was released from his contract criticising the chairman for not taking advantage of the government's furlough scheme, calling it "morally shocking".
[39] After leaving Ross County, Foster signed a one-year deal with then Scottish League One side Partick Thistle.