Prentiss was born Paula Ragusa in San Antonio, Texas, the daughter of Paulene (née Gardner) and Thomas J. Ragusa, a Social Sciences professor at San Antonio's University of the Incarnate Word, who was of Sicilian descent.
Before high school, Paula, who grew to 5 ft 10 in (1.78 m), was always the tallest person in class. She attended Lamar High School in Houston, Texas. In 1958 while studying drama at Northwestern University, she met future husband Richard Benjamin, who impressed her with his sophistication and height (he was taller than she was). While attending Northwestern she was discovered by Metro-Goldwyn-Mayer and was offered a film contract.
Prentiss first became known as a comedic actress in the early 1960s with such films as Where the Boys Are, The Honeymoon Machine, Bachelor in Paradise, and The Horizontal Lieutenant. In all four films Prentiss was paired with the 6 ft 5 in (1.96 m) actor Jim Hutton, as they were the two tallest male and female contract players at Metro-Goldwyn-Mayer.
She continued to star in comedies later with Rock Hudson in Man's Favorite Sport?, in The World of Henry Orient and What's New, Pussycat? with Peter Sellers, in Last of the Red Hot Lovers with Alan Arkin, in Move with Elliott Gould, and in director Billy Wilder's last film, Buddy Buddy, with Jack Lemmon and Walter Matthau. She also appeared in the war films Catch-22 and In Harm's Way, the thriller The Parallax View with Warren Beatty and the horror cult-classic The Stepford Wives, based on Ira Levin's novel.
For one season (1967–1968), Prentiss co-starred with her husband, Richard Benjamin, in the CBS sitcom He & She. Prentiss was nominated for an Emmy Award for Best Performance by an Actress in a Comedy.
Except for brief cameo roles, Prentiss had not appeared in a feature film for more than 30 years until 2016's I Am the Pretty Thing That Lives in the House, a horror film directed by Oz Perkins that premiered Sept. 10, 2016 at the Toronto International Film Festival.
Prentiss and actor/director Richard Benjamin were married on October 26, 1961. In 1965, while filming What's New, Pussycat in Paris, Prentiss suffered a nervous breakdown that kept her out of film work for five years. She returned to work with Benjamin in the short-lived television series He & She. The couple appeared together in such films as Catch-22 (1970) and Saturday the 14th (1981), the made-for-television films No Room to Run (made in Australia) and Packin' It In, as well as in various plays. Benjamin also directed Prentiss in a brief appearance in Mrs. Winterbourne.
Prentiss and Benjamin have two adult children, Ross and Prentiss. Ann Prentiss was Paula's younger sister and also an actress.