 Though some sources refer to him as the nephew of the 11th Baron Howth, the weight of evidence is that he was the only surviving son of Nicholas St Lawrence, 11th Baron Howth and Jane Montgomery, only daughter of George Montgomery, Bishop of Derry and his first wife Susan Steyning.He was born, probably in Colchester, Essex, in 1628.The St Lawrence family had inherited estates near Colchester from William's grandmother Elizabeth Wentworth, which presumably explains why he was sent to Colchester Grammar School, which he entered in 1639.