Eric Millegan grew up in Springfield, Oregon. He moved to Michigan to attend the Interlochen Center of the Ars, and later graduated with a musical theater degree from the University of Michigan.
Millegan has been performing on stage since he was seven years old, initially appearing in productions staged by Eugene Opera. In 2000, he made his Broadway debut, appearing in Jesus Christ Superstar. He also appeared in stagings of Madame Butterfly, Hansel and Gretel and The Pirates of Penzance.
Millegan has also appeared in several films, making his full-length debut in the 2002 indie drama On_Line. In 2005, he joined the cast of the Fox series Bones, appearing as prodigy Zack Addy.