Harmon was born in Flushing, New York. A runner-up for Miss Connecticut, the 5 ft 5 in, 41½-22-36 Harmon was a regular on Groucho Marx's television program Tell It to Groucho (credited as 'Patty Harmon'), and also guest-starred on several 1960s TV series, including Gidget, Batman, and The Monkees. She appeared in a cameo role as blonde Ardice in the Jack Lemmon comedy, Under the Yum Yum Tree in 1963. She had a role as Tony Dow's girlfriend in the 1965-66 television soap opera, Never Too Young, a program geared to teenagers, featuring many popular musical groups of that era.

Harmon's best-remembered acting roles are as the thirty-foot-tall Merrie in the 1965 movie Village of the Giants (where she captures normal-sized Johnny Crawford and suspends him from her bikini top), and as the car-washing girl "Lucille" in 1967's Cool Hand Luke, with Paul Newman.

Harmon retired from Hollywood to marry and raise a family. Later, she started her own company, Aunt Joy's Cakes, a bakery located in Burbank, California. She also made the rounds of movie and sci-fi fan conventions, to meet fans and sign autographs.