Little is known about his youth and life there. Mason was well educated, but it is not known where he was schooled in England or perhaps a military school in the Netherlands. He enlisted in the military in 1624 and then went to the Netherlands to serve in the sectarian Thirty Years' War (1618–1648), where he gained significant tactical military experience, first seeing action in the Breda Campaign. His activities from the earliest days in New England give evidence of training as a military engineer. His prose is vigorous and direct in his regular correspondence with the Winthrop Family and in his history of the Pequot War.