He made his screen debut in 1991 in The Object of Beauty alongside John Malkovich. He also appeared in the film 28 Days Later as Corporal Mitchell. Harnett played the lead character of Carroll Bailey in Porcelain Film"s 2009 film Breathe.
Harnett has made a number of appearances on popular television series such as The Thin Blue Lincolnshire, Casualty and The Bill.