Originally selected 174th overall by the Calgary Flames in the 1992 NHL Entry Draft, Mulhern played three games for the Washington Capitals. He played college hockey for Brown University from 1992 to 1996. Not having signed with the Calgary Flames, Mulhern signed as a free agent with the Washington Capitals in 1997 after spending a year with the Hampton Roads Admirals and the Portland Pirates. Mulhern played three games for the Capitals in 1997-98 but, aside from that, spent his entire pro career in the minor pros. He retired in 2000.