Hagan was born in Austin, Texas. She has been acting since the age of six, when her mother first signed her up for musical theater. Hagan was first involved with theater companies such as Houston's Playhouse 1960, the Houston Grand Opera (where she sang in the children's chorus), the Crighton Playhouse, the Houston Music Hall and the Bitter Truth Theater. From there, she made her screen debut in the 1997 feature film Faith, where she played the title character at age 13; her TV debut came on an episode of the Calista Flockhart series Ally McBeal in 1999.