hoag was raised in north olmsted, ohio, near cleveland, where she was born. her father, john c. hoag, was a supervisor for the a&p store chain. she graduated from john marshall high school in 1949. a straight-a student and head majorette of the marching band, she appeared in numerous high school stage productions, already using the nickname "mitzi". after high school, hoag attended shimer college, then located in mount carroll, illinois, completing her degree in two years under the radically flexible university of chicago curriculum of robert maynard hutchins. she chose shimer for its small classes and the opportunity for horseback riding. because shimer was then closely affiliated with the university of chicago, she is sometimes reported as having graduated from there.