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.

On 25 August 2011, Mulgrew continued his set piece record by scoring a free kick in Celtic's 1–3 defeat to Sion in their second leg UEFA Europa League qualifier.
The following day, Mulgrew signed a contract extension at Celtic to keep him at the club until at least 2015.

On 17 September 2011 he was sent off against Rangers for a second bookable offence, a challenge on Steven Davis.
Mulgrew scored his first SPL goal for Celtic on 15 October 2011, a headed equaliser at Rugby Park as Celtic came back from 3–0 down to draw 3–3 with Kilmarnock.
On 7 April 2012, Celtic beat Kilmarnock at Rugby Park to confirm them as Scottish Premier League champions.
Mulgrew scored two and set up two in the 6–0 win.
On 29 April 2012, he scored Celtic's first goal in a 3–0 victory over Rangers.
His performances won him the SPL, Players' and writers' player of the year awards for 2011–12.
He also picked up Celtic's Player of the Year and Player's Player of the Year awards.

On 21 July 2012 Mulgrew was sent off for a deliberate handball in a pre-season friendly against Ajax at the Amsterdam ArenA. As a result, he missed Celtic's opening 2012–13 SPL game with his former club Aberdeen.

On 1 August 2012, Mulgrew scored the winning goal in Celtic's 2–1 victory against HJK Helsinki in the first leg of the Champions League Third Qualifying Round.

His contract with Celtic expired at the end of the 2015–16 season.
Mulgrew was offered the use of Celtic's training facilities during the early part of the 2016–17 season.
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.
On 25 August 2011, Mulgrew continued his set piece record by scoring a free kick in Celtic's 1–3 defeat to Sion in their second leg UEFA Europa League qualifier. The following day, Mulgrew signed a contract extension at Celtic to keep him at the club until at least 2015.
On 17 September 2011 he was sent off against Rangers for a second bookable offence, a challenge on Steven Davis. Mulgrew scored his first SPL goal for Celtic on 15 October 2011, a headed equaliser at Rugby Park as Celtic came back from 3–0 down to draw 3–3 with Kilmarnock. On 7 April 2012, Celtic beat Kilmarnock at Rugby Park to confirm them as Scottish Premier League champions. Mulgrew scored two and set up two in the 6–0 win. On 29 April 2012, he scored Celtic's first goal in a 3–0 victory over Rangers. His performances won him the SPL, Players' and writers' player of the year awards for 2011–12. He also picked up Celtic's Player of the Year and Player's Player of the Year awards.
On 21 July 2012 Mulgrew was sent off for a deliberate handball in a pre-season friendly against Ajax at the Amsterdam ArenA. As a result, he missed Celtic's opening 2012–13 SPL game with his former club Aberdeen.
On 1 August 2012, Mulgrew scored the winning goal in Celtic's 2–1 victory against HJK Helsinki in the first leg of the Champions League Third Qualifying Round.
His contract with Celtic expired at the end of the 2015–16 season. Mulgrew was offered the use of Celtic's training facilities during the early part of the 2016–17 season.
Charlie Mulgrew