Willis studied at the University of Arizona before completing her bachelor's degree at the University of New Mexico. She began her career creating sets and costumes at the University of New Mexico in Albuquerque. She went on to earn a master's degree at the Yale University Art and Drama School.
She was nominated for an Oscar seven times, earning the Academy Award for her colorful designs for The Wonderful World of the Brothers Grimm in 1962. In addition to designing for film, she has also worked on live shows, including Shipstead & Johnson's Ice Follies and the New Buffalo Bill Wild West Show.