Allen was born in New York City, New York, the youngest of three children of Eugene and Florence Allen. Her father was a police lieutenant in Yonkers,[1] where she was raised.

She was terribly shy as a child, so her mother enrolled her in dance classes at age 4. She attended the High School of Performing Arts, where she trained for a dancing career, and then attended Jose Quintano's School for Young Professionals.

Her film debut was when she was 12, playing the role as Melissa Dillon in "Money in My Pocket" (1962), a comedy/drama starring country singer Bill Anderson. From the time she was 15, Allen appeared in a large number of television commercials. She also worked as a model.

Allen began an acting and modelling career as a child, and from the mid-1970s appeared in small film roles. A strong supporting role in "Carrie" (1976) brought her recognition, and after marrying the director Brian De Palma, she appeared in several of his films, including "Dressed to Kill" (1980), for which she received a Golden Globe nomination, and "Blow Out" (1981). Her subsequent films include "The Philadelphia Experiment" (1984) and the "RoboCop" trilogy.