Mulgrew began his career with
Glentoran, making two senior appearances during the 2004–05 season. In 2005, he moved to Glentoran's rivals
Linfield and quickly became a fixture with the first team squad. He was named as Linfield's Player of the Season for 2010 and as of the end of the 2019–20 season had appeared 589 times for the club in all competitions, having scored 45 goals. It was reported by the
Belfast Telegraph on 8 February 2011 that
Scottish Premier League clubs
Kilmarnock and
Hibernian, along with
Major League Soccer club
Portland Timbers, were interested in signing Mulgrew on a
Bosman free transfer when his contract expires in the summer. In 2011, Mulgrew had trials with Major League Soccer sides
Columbus Crew and
Portland Timbers. He was named
Ulster Footballer of the Year and
Northern Ireland Football Writers' Association Player of the Year for the 2016–17 season. ==International career==