Monck became a professional soldier, a common career choice for younger sons of impoverished gentry. His first experience was the failed attack on Cádiz in November 1625, when he served as an ensign in a company commanded by his cousin Sir Richard Grenville. He later joined the equally disastrous expedition against St Martin-de-Ré in July 1627; it is suggested one reason for doing so was his arrest for attempted murder in late 1626, when he and his brother Thomas assaulted Nicholas Battyn, the Undersheriff responsible for jailing their father.