On 30 August 2009, Forrest signed a four-year contract with Celtic. Their Head of Youth development, Chris McCart, hailed him as one of the best players in Celtic’s U19 squad. Forrest had always excelled at Celtic, with a match program from 2004 singling him out for exceptional performances in the under-13 squad. He then made his first team debut for Celtic on 1 May 2010, coming on as a late substitute against Motherwell. He scored the third of Celtic’s goals in a 4–0 win.
He made his first European appearance for the club in a Champions League qualifying game against Braga in July 2010. A few days later he won the man of the match award for his performance in the Emirates Cup game with Olympique Lyonnais. He continued this fine form into the league campaign, scoring against St Mirren in a 4–0 victory. Forrest was named the SPL Young Player of the Month for August 2010. He would score another two goals, both against Hearts, before being rewarded for his form with a new five year deal on 28 January 2011. Forrest collected his first winner’s medal on 21 May 2011 when he came on a late second-half substitute for Kris Commons in Celtic’s 3–0 Scottish Cup Final win over Motherwell.