Kelly is best known for his role playing Kev Ball in Channel 4's Shameless. Other television credits include Maisie Raine, Border Cafe, Tripping 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 January 1, 2007, he starred in ITV Drama Dead Clever alongside Suranne Jones. He left Shameless after the first eisode of the fourth season 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 scifi 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 season. In January and February 2009, he appeared in two episodes of Being Human as a werewolf named Tully.
He starred in Frequently Asked Questions About Time Travel a comedy sifi film with Chris O'Dowd, Marc Wootton and Anna Faris, ITV drama Married Single Other with Ralf Little and Miranda Raison, the BBC miniseries Cranford as a poor man struggling to feed his six children and wife, and in Robin Hood as Robin's father, Malcolm.
In 2002 he starred as Private Willie McNess in the major motion picture Deathwatch, a 2002 European horror film directed by Michael J. Bassett.
He narrated a documentary style DVD about the Great Britain Rugby League team during the 2005 Rugby League Tri-Nations titled League of Their Own.
In 2009 he starred alongside his brother in the ITV1 drama Collision which ran over five consecutive nights in November. In 2010 he played industrialist Christoper Rawson in BBC drama The Secret Diaries of Miss Anne Lister. In 2013 he played Ian, the boyfriend of the title character, in the BBC One drama series Frankie. In 2013 he took on the role of Danny, a soldier facing his fears in the short film On the Bridge, based on a true story and shot entirely on Waterloo Bridge.
Empire had announced that Dean would be voicing the role of John Lennon for the then-upcoming 3D remake of Yellow Submarine before the project was cancelled. However in late 2011, Kelly appeared as the narrator on the interavtive Apple iBooks version of Yellow Submarine.