[18][19] 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.

[22] In Celtic's Scottish League Cup quarter-final tie with Hibernian on 26 October, Forrest was credited with a match-winning performance.

[28] He scored against St Mirren and Dunfermline[29][30] before his season ended after suffering an injury during Celtic's defeat to Kilmarnock in the 2012 Scottish League Cup Final.

[38][39] Forrest finished the season with a second consecutive league title and his second Scottish Cup winners medal after starting Celtic's 3–0 win against Hibernian in the final.

[40] In July 2013, Forrest scored his first European goal for Celtic in a second round Champions League Qualifier against Cliftonville.

[45][46] Forrest was arrested by police in November 2013 over allegations of indecent exposure and sexual assault at a nightclub in Prestwick.

[48][49][50] He scored 7 goals and assisted 5 in only 27 appearances in another injury dominated season as Celtic won three league titles in a row.

[52][53] Two of Forrest's three European appearances came in the Europa League including coming on as a substitute in the second leg of Celtic's 4-3 aggregate defeat to Inter Milan in the last 32 of the competition.

He featured more often in the Spring and came on as a substitute in Celtic's 2–0 victory against Dundee United in the Scottish League Cup Final, scoring the second goal before earning his side a penalty which he would fail to convert.

[55][56] Forrest made 29 appearances and scored and assisted 4 goals as Celtic won the league for the fourth consecutive season.

On 27 November 2016 Forrest scored the second goal and won a penalty in a man of the match performance in the League Cup final as Celtic defeated Aberdeen 3–0 to win the club's 100th trophy.

[69] Forrest made 46 appearances and scored 8 goals and assisted 13 as Celtic finished the domestic season without defeat after 47 matches to be nicknamed the "Invincibles".

The beginning of the 2017–18 season saw Forrest play in every Champions League qualifier as he helped Celtic qualify for the group stage by scoring the decisive goal away to Rosenborg in a 1–0 aggregate victory in the third round and he scored another goal in an 8–4 aggregate win against Astana in the play-off round.

[86] Forrest began the 2018–19 season as first choice right-winger and he scored in both legs in the first round of Champions League qualifiers against Alashkert.

He conceded a penalty and was sent off as last defender in a 3–1 Europa League group stage defeat away to FC Red Bull Salzburg on 4 October 2018,[87] at which time he was yet to score in the domestic campaign.

However he kept his place for the next league fixture against St Johnstone at McDiarmid Park and responded positively, scoring four goals (the first Celtic player in six years to do so)[88] in the first half of an eventual 6–0 win.

[90] In November he helped Celtic progress to the knock out stage of the Europe league by providing assists in wins against RB Leipzig and Rosenborg.

[98] On 14 April 2019 Forrest opened the scoring against Aberdeen in the Scottish Cup semi-final with a long range strike.

[100] He started Celtic's 2–1 victory against Hearts in the Scottish Cup final to complete a historic third consecutive treble.

[110] and in the Europa League he won a penalty to help Celtic earn a 1–1 draw in their opening match against Stade Rennais.

Forrest made his return as a substitute in a 0–0 draw with Dundee United which saw Rangers confirmed as league champions.

On 20 November 2021 Forrest came on as a substitute and scored the only goal of the game as Celtic defeated League Cup holders St Johnstone in the semi-final of the competition.

[133] On 17 December 2022, Forrest became the third player, along with Jimmy Johnstone and Henrik Larsson,[134] for Celtic to reach both 100 goals and assists for the club, when he set up Callum McGregor in a 1-0 against Aberdeen at Pittodrie.

On 15 December 2024, Celtic won the 2024 Scottish League Cup final against Rangers, giving Forrest his 25th trophy.

[141] On 7 September, he came off the bench for Scotland U21s and assisted in the 89th minute for Chris Maguire to score the winning goal against Austria.

[142] On 17 May 2011, Forrest was selected in Craig Levein's Scotland senior team to face Wales and the Republic of Ireland in the Nations Cup.

[145] On 22 December 2011, it was revealed that Forrest had been invited to the Great Britain team for the 2012 Summer Olympics,[146] but did not make the final cut for the Games.

On 14 June 2021, he made his only appearance in the tournament as a substitute in Scotland's opening game 2–0 defeat to the Czech Republic at Hampden Park.

[153] He did not feature for Scotland over the next few years, but a run of form for Celtic in spring 2024 led to a recall for the UEFA Euro 2024 provisional squad.

[157][158][159] Former Rangers winger, and Scotland under-21 teammate, Gregg Wylde described him as being the kind of player who never hides from his responsibility during a match.