Millegan grew up in Springfield, Oregon. He studied acting at the Interlochen Arts Camp at Interlochen Center for the Arts in Michigan before studying musical theatre at the University of Michigan. He currently lives in Los Angeles, California.
In 2003, Out Magazine named him "Hottest Up-and-Coming Openly Gay Actor of 2003."
Many of Millegan's credits are in musical theatre. Millegan began his musical theatre career at age 7 in the 1981 Eugene Opera production of Menotti’s Amahl and the Night Visitors and continued with the company in several other productions including Madame Butterfly, La Boheme, Carmen, and Hansel and Gretel. Millegan appeared on Broadway in the 2000 production of Jesus Christ Superstar. In January 2005, he starred as Harold opposite Estelle Parsons' Maude in the world premiere of Tom Jones and Joseph Thalken's Harold & Maude: The Musical at the Paper Mill Playhouse in Millburn, New Jersey. In 2006 he returned to Eugene, Oregon to play Frederic in Eugene Opera's production of The Pirates of Penzance.
Eric is a fan of the Portland Trail Blazers.
Eric has three brothers - Jeffrey, Brantly, and Patrick - and two sisters - Lisa and Colleen.
He made his film debut in On Line (2002).
He has appeared on Law & Order: Criminal Intent, 100 Centre Street, and Curb Your Enthusiasm.
Millegan has been cast to play the lead role in The Phobic directed by Margo Romero and produced by Alex Ryan Productions Inc.
Millegan also performs in his own cabaret show entitled, "Don't Tell Mama", a show he began doing early 2000. In 2004 following his performance, he decided that he was going to perform said cabaret show every four years.
On February 26, 2008, he performed the National Anthem at the L.A. Lakers vs Portland Trail Blazers NBA playoff game.