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.
On the outbreak of the Civil War Merrick was appointed Sergeant-Major-General in the Parliamentary Army commanded by the Earl of Essex. He commanded an infantry regiment Sir John Merrick's Foot, which took part in the Battle of Edgehill. After the Battle of Stratton, Merrick was appointed general of the Artillery, leading them at the Battles of Gloucester and Newbury.
In April 1640, Sir John was elected Member of Parliament for Newcastle under Lyme in the Short Parliament. He was re-elected MP for Newcastle under Lyme for the Long Parliament in November 1640. In 1647, he served as assessment commissioner for Pembrokeshire. However, his disapproval of the execution of the king in 1649 led to his retirement during the Interregnum.
- BBC - Parliamentary Generals in the 1640s - Sir John Merrick
- Willis, Browne (1750). Notitia Parliamentaria, Part II: A Series or Lists of the Representatives in the several Parliaments held from the Reformation 1541, to the Restoration 1660 ... London. pp. 229–239.
- "Meyrick or Merrick family". Dictionary of Welsh Biography. National Library of Wales. Retrieved 5 September 2017.