On 1 July 2010, Mulgrew returned to Celtic on a three-year deal, becoming new manager Neil Lennon's first signing. On 2 January 2011, having played only a handful of games, he played against Rangers in a 2–0 win at Ibrox on the left wing. On 29 January, he deputised for Daniel Majstorović at centre back against former team Aberdeen in a 4–1 League Cup semi-final win at Hampden Park. He scored his first goal for Celtic in that game, nodding home Kris Commons' corner from close range. Mulgrew became a regular in the centre of defence alongside Majstorović. In the Scottish Cup semi-final on 17 April, he scored his second goal for Celtic from a long-range free kick; like his first goal, it was at Hampden Park and against Aberdeen. On 21 May 2011, Mulgrew kept up the trend of scoring at Hampden by netting in the final against Motherwell. He scored the final goal of Celtic's 3–0 win with a free kick into the top corner, and helped earn him his first medal in football.