 William was the younger son of the Dublin-born poet and historian Richard Stanyhurst and his second wife Helen Copley, who died during his infancy, and was born at Brussels on 15 June 1601.After studying at the Jesuit college in Brussels, he entered the Society of Jesus at Mechelen on 25 September 1617.His older brother Peter had joined the Jesuits a year before him, and died while serving as a chaplain to the Flemish fleet on 22 April 1627.