John Merrick joined the 3rd earl of Essex's expedition to Flanders in 1620. In 1624, he fought as a soldier in the Low Countries and fought in Spain in 1625; he was also knighted that year. He fought in the Thirty Years' War under Gustavus Adolphus and was wounded on 17 August 1632 before Maastricht. In 1639-40, he commanded a regiment in the Bishops' Wars. When the Irish rebellion broke out in October 1641, the Commons recommended Sir John for a commission in Ireland.