 Prendergast was born in Abertridwr, a mining village in the Aber Valley near Caerphilly in Glamorgan.His father was a Roman Catholic from Wexford, in Ireland who sought work as a coal miner in Maesteg in south Wales after the 1916 Easter Rising.His older brother (Stewart) and his twin (Paul) attended the local grammar school, but he was sent to the local secondary modern, where his art teacher, Gomer Lewis, recognised his artistic talent.