Smith stumbled upon acting when he accompanied a friend to an audition for the 1989 film A Nightmare on Elm Street 5: The Dream Child. Smith landed the role of Super Freddy. Smith would appear three years later in a small role in Renegade playing PJ Butler. In 1994 he appeared in a Marvel Comics adaptation of The Fantastic Four as Ben Grimm. Smith was also cast in Cyborg 3: The Recycler alongside Malcolm McDowell.
Smith appeared in many TV series, such as Diagnosis: Murder, Star Trek: Voyager, Wings and Conan the Adventurer. In 1999 Smith had small roles as guards in Donald Petrie's My Favorite Martian and The X-Files. Smith appeared 18 times in the hit TV show Charmed, where he played Belthazor, The Source, Grimlock and Shax. 2002 also saw him appear in the hit sequel Men in Black II, where he portrayed the character Creepy. After 2002 he appeared in the TV series The O.C. and Desperate Housewives. In 2003 Smith was brother Bob (Monster Man) in the hit movie Monster Man.
In 2006 Smith was cast as villain Pluto in the remake of The Hills Have Eyes. In 2007, he played villain Papa Hades in The Hills Have Eyes 2. He stars also in the 2009 horror film Chain Letter, alongside Nikki Reed, and Noah Segan, directed by Deon Taylor.