Donnelly's career in television began at the age of 12, when his father suggested that he audition for "Byker Grove", a new children's television programme being created by CBBC in Newcastle upon Tyne that had been advertised in local newspapers. His success in this audition led to him securing the role of Duncan, whereupon he first met Anthony McPartlin. Although he was raised in an estate close to Donnelly, McPartlin had never met him until their involvement in the children's drama. The pair worked together on the show until their eventual departure in 1993, whereupon they began to bond as friends, both socially and professionally.