Thompson signed for Celtic on 8 August 2009 for a £250,000 fee, teaming up with Celtic Tony Mowbray who had taken Thompson on trial while he was manager of West Brom. Thompson signed alongside fellow former Stockport youth team players Curtis Jones and Michael Ordish. He made his debut coming on as a substitute for Stephen McManus at Celtic Park in a 1–1 draw with Falkirk on 16 January 2010. He made his first Celtic start in a 4–1 away victory against St Johnstone on 20 March 2010 and scored his first goal for the club in a 3–0 victory over St Johnstone on 20 March 2010. Following the match and his first professional goal, Thompson said he wasn’t expecting to score for the club so soon after joining. He followed his debut goal up with two in a 2–1 victory for Celtic against Motherwell on 13 April 2010. Despite limited first-team opportunities at Celtic, Thompson was named as the club’s Young Player of the Year in his first season. Upon receiving the award, Thompson said that since joining Celtic, he had changed his attitude both on and off the pitch. He made 21 appearances in all competitions for Celtic during the 2009–10 season, and played his final game of the campaign in a 2–1 away victory over Hearts on 9 May 2010. This was to be his final game for the club.
Joshua Thompson was born in Bolton, England on 25 February 1991 (age 31).