After Martin O’Neill became the new manager of Celtic in June 2000, he made several attempts to sign Lennon. Finally after months of protracted negotiations with Leicester City, on 6 December 2000 Lennon joined the club he supported as boy for a fee of £5.75 million. That season saw Lennon pick-up three winner’s medals as Celtic swept to a domestic treble, winning the Scottish Premier League, Scottish Cup and Scottish League Cup. The next four seasons at Celtic saw Lennon win a further two league championships and two Scottish Cups. He was also an integral part of the Celtic side that reached the UEFA Cup Final in 2003, losing 2-3 to Porto after extra time.
When Gordon Strachan took over from Martin O’Neill as Celtic manager in the summer of 2005, he made Lennon Celtic captain. In January 2006 when his former club Leicester City sacked Craig Levein, Lennon was linked with a return to the Midlands side in a player-manager role. He went on record saying he was flattered but wanted to captain Celtic to the Scottish Premier League title. Lennon picked up his fourth League Championship medal that season on 5 April 2006 when Celtic defeated Hearts 1-0 at Parkhead.
Although he was also linked with a move to Crystal Palace in a player-manager role in the summer of 2006, on 23 June 2006, Celtic announced he had signed a new one-year contract. Sunderland manager Roy Keane made an attempt to sign Lennon prior to the closure of the August 2006 transfer window, but his approach for the player was rejected by Celtic. On 25 April 2007, Lennon announced he would be leaving Celtic, and in his last game for the club on 26 May 2007, he captained the team to victory in the Scottish Cup Final against Dunfermline Athletic. Celtic’s 1–0 win clinched the League and Cup Double.