Hennesy began his movie and television career as a stuntman and actor, while he was in college and playing football at USC.
Hennesy served in the United States Navy in World War II.
Hennesy was the stunt double for many stars, including Rock Hudson, Randolph Scott, Rod Cameron, Jeff Chandler, and more.
Hennesy also worked as a general secondary, as well as elementary school, teacher in many Hollywood studios.
His celebrity students included, among others, Natalie Wood, Annette Funicello, Paul Anka, Sal Mineo, Tim Considine, Molly Bee, Tommy Rettig, and Lauren Chapin.