Her father was the chairman of the Department of Physical Education at California State University, Northridge (CSUN) and her mother was an English teacher and college counsellor at Monroe High School.
She credits her first interest in acting to seeing an interview with Kym Karath (who played Gretl in The Sound of Music on Art Linkletter's television show House Party when she was five or six years old.
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.
