Donnelly was born in Bournemouth on 28 October 1985, the son of Chrissie Wickham and Tony Donnelly. His mother was the choreographer of Bucks Fizz's winning dance routine at the 1981 Eurovision Song Contest, as well as an actress and former Hot Gossip dance troupe member. He was raised primarily in Ringwood, Hampshire. He attended St Catherine's Primary School in Wimborne Minster, Dorset, and then St Peter's School in Bournemouth. He later joined the Big Little Theatre School in Bournemouth.