Nothing is known of John Felton's life until the mid-1620s, when he was an army officer. He served in the Cádiz Expedition of 1625, an attempt to capture the Spanish city of Cadiz that was backed by Buckingham. This resulted in a decisive Spanish victory, with 7,000 English troops and 62 out of 105 ships lost. Felton then served as a lieutenant in Ireland in 1626, during which time his commanding officer died and Felton tried, but failed, to be appointed as his replacement.