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.

[1] 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.

[2] In 2011, Mulgrew had trials with Major League Soccer sides Columbus Crew and Portland Timbers.

[4][5] He has been capped at the under-21 level for his country, playing against Moldova in a UEFA Under-21 European Championship qualifying match.