That was over 30 years ago in 1990.
Today she is 67, and has starred in 70 movies in total, 53 since Back to the Future Part III was released.
In Back to the Future Part III, I think Mary Steenburgen looks:
Back to the Future Part III scores 7.2 out of 10 on The Movie Database.
Mary Steenburgen's first acting role was as 'Julia Tate / Moon' in Goin' South, released in 1978 when she was 25.
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