Hewlett was born in Redhill, Surrey, United Kingdom (age 46), and moved with his family to Canada at an early age. Doctor Who sparked his love for science fiction and in England he made sci-fi on his 8-millimetre camera with friends.
In his mid-teens he became a self-described "computer nerd", and also launched his acting career by starring in student films directed by school friend Vincenzo Natali. He dropped out of high school and, with his sister Kate Hewlett - also an actor - ran Darkyl Media, a web site design firm, and also founded Fusefilm.com, an internet community for filmmakers.
Hewlett appeared in many low-budget horror movies, such as The Darkside (1987) and minor cult favourites Pin (1988) and Scanners II: The New Order (1991), and in 1997 worked again with Vincenzo Natali in the critically acclaimed thriller Cube. He also guest starred in several television series such as Kung Fu: The Legend Continues (1993-96), Traders (1996-2000).
In 2002, he began a recurring role on the Canadian-American sci-fi series, Stargate SG-1 as Dr. Meredith Rodney McKay with his appearances proving popular with fans. In 2004, Stargate SG-1 launched a spin-off series titled Stargate Atlantis, a show on which Hewlett became a member of the main cast and which is his best known role to date.
In 2006, Hewlett wrote and directed the dark comedy feature film A Dog's Breakfast starring himself, Kate Hewlett, his dog Mars, and Stargate: Atlantis co-stars Paul McGillion, Christopher Judge, and Rachel Luttrell. This movie also marks Hewlett's own directional debut. In 2007, he guest starred in the first episode of the TV series Sanctuary, produced by and starring Stargate actress Amanda Tapping. He has just finished directing his second movie titled Debug, scheduled for release in November 2014.