Cassidy is the son of stuntman and stunt coordinator Michael Cassidy and the brother of stunt performers Zane Cassidy and Mariah Cassidy. Because of his fathers connection to the stunt business, Cassidy got at early age into the movie and television business. His first experiences were the science fiction fantasy film The Ewok Adventure (1984, with Fionnula Flanagan and Darryl Henriques, stunt coordinated by his father and with stunts by Frank James Sparks) and the family comedy Honey, I Shrunk the Kids (1989, with Matt Frewer, Amy O'Neill, Mark L. Taylor, Frank Welker, Craig Richard Nelson, and stunts by Cherie Tash). For the letter one he performed three high fall stunts and finally earned his SAG card. Cassidy doubled actor Macaulay Culkin on most of his '90s films and set a world recor