Hagan was born in Minneapolis, Minnesota, the daughter of Mary Elizabeth (née Henslee) and John Robert Hagan. She was raised in Fort Wayne, Indiana.
Hagan has made appearances in films such as Some Kind of Wonderful and most notably as the young Ellie Ewing in the acclaimed TV movie Dallas: The Early Years (a prequel of the long running TV series Dallas) and she has appeared in several other TV shows. She was in two Columbo episodes. In the end of the second season of Star Trek: Deep Space Nine, she played a young Vorta called Eris. Another notable role was as Angel in seminal '90s Fox sitcom Herman's Head. She also appeared on Seinfeld as Sister Roberta, the nun that Kramer nearly took from the church. She played a trailer trash mother in a Jerry Springer-based movie, Ringmaster. She played Emma Roberts's mother in the Nickelodeon series Unfabulous from 2004 to 2007. She also made a guest appearance as Karen Young in The WB television series Charmed in the episode "The Fifth Halliwheel." On Friends, she played a cooking class instructor named Sarah.