Kelly is best known for his role playing Kev Ball in Channel 4's Shameless. Other television credits include Maisie Raine, Border Cafe, Tipping the Velvet, A Midsummer Night's Dream and The Worst Week of My Life. He also had parts in the films The Lowdown and Mike Bassett: England Manager. In 2006 he took a lead role in BBC One's drama series Sorted, which is about a group of postmen. On 1 January 2007 he starred in ITV Drama Dead Clever alongside Suranne Jones. He left Shameless after the first episode of the fourth series along with his co-star and on screen wife, Maxine Peake. In 2007, he appeared as william Shakespeare in an episode of the BBC One science-fiction series Doctor Who. In March 2008, he appeared as one of Jesus' disciples, James, in the BBC drama The Passion, a retelling of the last days of Jesus' life. In May, he appeared as Hedley, the landlord of the local pub in BBC drama The Invisibles, a story of two retired crooks returning to Britain from abroad. Hedley then joins the twosome later in the series. In January and February 2009, he appeared in two episodes of Being Human as a werewolf named Tully.