Born as Spencer Garrett Heckenkamp in Los Angeles, California to actress Kathleen Nolan and former head of the talent agency Film Artists Associates Richard Heckenkamp, he attended Duke University and Fordham University. He studied acting with Sanford Meisner at The Neighborhood Playhouse, Jeff Perry, Robert Neff Williams at The Juilliard School, and Alfred Molina. In 1996 he received a Dramalogue Award for Best Actor in the stage play "Heartbreak", performed at the Hudson Guild Theatre.
In 2007, Garrett was part of the ensemble of the drama Bobby' and received a Screen Actors Guild Award nomination for Best Ensemble Cast. Garrett's career in front of the camera began around the year 1989, when he appeared in a made-for-television film called Love and Other Sorrows, a production that also featured DS9 guest actor Tim Ransom. Garrett also had a brief role in the 1996 drama Ghosts of Mississippi which also starred Whoopi Goldberg, Susanna Thompson, Virginia Madsen, Brock Peters, Terry O'Quinn, Jerry Hardin, Bill Cobbs, Richard Riehle, Jordan Lund, and Thomas Kopache. Since then, Garrett has had numerous television and some film appearances in supporting roles. Garrett has been working as a producer. In cinematic film, he played the Air Force Chief in Transformers: Revenge of the Fallen. Some other Star Trek personalities had similar military roles, with Glenn Morshower and Eric Pierpoint playing a general and NSA officer, respectively. Robert Foxworth, Tony Todd and Frank Welker also provided voices for the movie. In 2013, he played the small-town sheriff in Iron Man 3 (with William Sadler, Miguel Ferrer, and Tom Virtue) and Andy Hayes in the Mad Men episode "A Tale of Two Cities" (with Gary Werntz).