The 26-year-old, a product of the Hoops' youth academy, has been one of Celtic's key players since Neil Lennon brought him back to the club in 2010.
Speaking to the official Celtic website, Mulgrew said: "It's good to get it signed and focus only on the football. Last year was a good one for me. I've been happy to be back. Things have been going well for me so hopefully that can continue."
The Scotland defender has become a favourite of the Celtic fans and he is looking to bring them continued success.
He said: "It humbles me to hear them singing my name and getting behind the team.
"Everyone appreciates it and it helps us. I know it's a cliché but they are the 12th man at times and it really helps the team.
"All the players here now understand what this club is all about, and for me, growing up as a Celtic fan, I understand fully what the fans expect here and hopefully we can deliver that.
"All the boys have a big drive and they've been here long enough to understand what the club is all about.
"It's a massive club and there is a big pressure here, week in and week out, but everybody's happy here.
"That's what you've got to do at a club like Celtic - the fans expect things, they expect you to win games, so I know the expectation levels here and hopefully we can continue doing well and winning games for Celtic."
Source: PA
Source: PA