The lure of joining Celtic, the club he had supported as a boy, persuaded McCourt to move to Glasgow and, on 19 June 2008, he signed for a fee of £200,000 after agreeing a three-year contract. The manager who signed him, Gordon Strachan, would later claim that McCourt was “as gifted a footballer as I have ever seen.”
McCourt made his competitive debut for Celtic on 25 October 2008 in a 4–2 win over Hibernian. In his first season, he featured primarily for the Celtic reserve team, playing in the deciding game on 28 April 2009 against Rangers reserves to clinch the Reserve League Title for the Hoops for the eighth season in succession.
On 23 September 2009, McCourt opened his goalscoring account for Celtic with a memorable goal in a League Cup match at Falkirk, in which he beat five defenders before chipping the ball over Bairns goalkeeper Bobby Olejnik. He then scored his first SPL goal on his first league start three days later against St Mirren, which saw him go past six defenders before beating the goalkeeper. On 24 January 2010, McCourt bagged his second league goal for Celtic against St Johnstone after a mazy run and finish into the top right hand corner, from just inside the penalty box. English Premier League newcomers Blackpool expressed an interest in McCourt following his performances throughout the 2009–10 season, prompting Celtic to offer him an improved contract, which he duly signed.
In the first game of the 2010–11 SPL season, McCourt scored the only goal of the game against newly promoted Inverness CT with a now-typical solo run and shot, beating three defenders and then netting from the edge of the box. He scored his second goal of the season – his first at Celtic Park – in Celtic’s 3–0 victory over Hearts on 11 September 2010, beating three players before chipping the goalkeeper. On 6 November 2010, McCourt netted the final goal of Celtic’s 9–0 demolition of Aberdeen. Later that month, he scored Celtic’s 600th home SPL goal in a 2–2 draw with Inverness at Celtic Park, with another jinking run and precise finish. On 29 December 2010, McCourt scored the winning goal in a 1–0 win against Motherwell. On 26 January, McCourt added another SPL goal in a 4–0 win over Hearts. He came on as a substitute in the 2011 Scottish Cup Final on 21 May 2011, a 3–0 win over Motherwell, to win his first silverware in Scotland. On 15 August 2011 it was reported that several English clubs, including Liverpool, were interested in buying him.
McCourt was mainly used as a late substitute over the course of the next two seasons, failing to score a single goal, though he was part of the Celtic team that won both the 2011–12 and 2012–13 SPL titles, as well as the 2013 Scottish Cup. In the final at Hampden Park, a 3–0 victory over Hibernian, he entered play as a substitute and was handed the captain’s armband in what proved to be his final appearance for the club. McCourt left Celtic after his contract expired in June 2013, having played in a total of 88 games, scoring ten goals. On 6 August 2013, he began training with Hibernian to try and win a contract, but no move materialised.