Dru was born in Logan, but her family later lived in Clarksburg and Wheeling, where she went to high school before moving to New York City at age 16.

She worked as a model and dancer before moving to Hollywood with her first husband, singer Dick Haymes, in the early 1940s. Her film career began in 1946, and she starred in classic Westerns such as Red River and She Wore a Yellow Ribbon alongside John Wayne. She acted in over 40 films and later appeared on TV shows like Guestward Ho!

She was singer and game-show host Peter Marshall's brother.