Winningham attended Andasol Ave. Elementary School, where her favorite activities included drama and playing the guitar and drums. She took the extended drama option at Patrick Henry Junior High School and continued to study over her summer vacations at CSUN's Teenage Drama Workshop. It was at this time that she adopted the nickname "Mare". Her mother arranged for her to go to Chatsworth High School. In grade 12, Winningham starred in a production of The Sound of Music, playing the part of Maria, opposite classmate Kevin Spacey as Captain Von Trapp. She graduated co-valedictorian (with Spacey) of her high school class in 1977.