Billing has starred in many television shows and had main roles on Bad Cop, Bad Cop, Dossa and Joe and Hell Has Harbour Views. Billing also had a recurring role on Blue Heelers as Senior Constable Ian Goss, on Always Greener as Eddie McGill, in Packed to the Rafters as Ron Barrett, and in Under the Mountain. In 2000 he featured in the film The Dish, portraying the Mayor of Parkes in a comedy about the moon landings. He also appeared as part of the ensemble on the 2006 season of Thank God You're Here. He has had a guest role on All Saints.

In 2009, Billing played Griffith drug lord Robert Trimbole in the high-rating drama series Underbelly: A Tale of Two Cities, starring alongside Matthew Newton. He also starred in the film Charlie and Boots. Roy played the part of Chief Dufflepud in the upcoming 2010 film The Chronicles of Narnia: The Voyage of the Dawn Treader. He will play a recurring character in the upcomng Nine Network police drama Cops L.A.C.. An early appearance was in Sea Urchins, a 1980 New Zealand TV film.