Horne was born in Bradford, Pennsylvania, but moved with her parents to Long Beach, California, when she was 11. At the age of 13, she became part of the newly formed Los Angeles Concert Youth Chorus. She attended the University of Southern California where she was a member of Pi Beta Phi fraternity. She is an alumna of Long Beach Polytechnic High School and returned in 1995 in a performance to benefit its music program. As a high school student, Marilyn was part of the St. Luke's Choir of Long Beach under the direction of William Ripley Dorr. This prestigious choir often worked for the movie studios and recorded on Capitol Records. Marilyn and her sister Gloria were part of St. Luke's Quartet along with tenor, Bob James and Baritone, Philip D. Haynes.

She studied voice under William Vennard at the University of Southern California School of Music and participated in Lotte Lehmann's vocal master classes.