Detmer was born in the bay area and raised in Chico, California to Susan, a reading teacher, and Melvin Detmer, a singing cowboy. She attended California State University, Chico for her undergraduate education; deciding to complete her Master of Fine Arts from NYU's Tisch School of the Arts before seriously embarking on a career in show business.

She made her onscreen debut in 1995 with the television movie Stolen Innocence, and her big screen debut playing Miss Minneapolis in the beauty pageant-set comedy Drop Dead Gorgeous. She has also made notable film appearances in Final Destination as Terry Chaney, Boys and Girls, Saving Silverman, The Majestic and Big Fat Liar. In television, she also appeared in the 2006 made-for-TV movie Proof of Lies and the ABC dramas What About Brian (as Deena) and Private Practice (as Morgan).