Willis went up to Christ's College, Cambridge in 1631, and was admitted to Gray's Inn in the same year. As a younger son, he stood to inherit little, so became a career soldier, enlisting in the Dutch military and serving at the Siege of Breda in 1637, returning to serve under King Charles I in the Bishops' Wars from 1639 to 1640. Prior to the outbreak of civil war, Willis was a member of the King's Guard based at Whitehall under Sir Thomas Lunsford.