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.
In June 2011, Mulgrew stated that he would consider playing for the United States, whom he was qualified to play for due to an American-born grandmother, if he was continually overlooked for Scotland. In February 2012, Scotland manager Craig Levein handed Mulgrew his first call-up to the senior squad, and he made his international debut on 29 February, starting at left back in a 1–1 draw with Slovenia. He also played in a 5–1 defeat to the USA in Florida in May 2012.