He then served as trial lawyer (admitted in 1604 as a student at Inner Temple), soldier (possibly in the Low Countries), sailor, and author (pupil of John Davies of Hereford) before turning to piracy.
In 1610, at the age of 24, Mainwaring was given a commission from Lord High Admiral Nottingham to capture the notorious Newfoundland "arch-pirate" Peter Easton, then feared to be hovering around the Bristol Channel.
On 4 June 1614, off the coast of Newfoundland, Mainwaring, in command of eight vessels, plundered the cod-fishing fleet, stealing provisions and taking away with him carpenters and mariners.
Sailing to the coast of Spain, Mainwaring then took a Portuguese ship and plundered her cargo of wine, and he later took a French prize and stole 10,000 dried fish from her hold.
When Mainwaring was away from his main base at La Mamora, on the Atlantic coast of present-day Morocco, a Spanish fleet under Admiral Luis Fajardo, sailing from Cádiz on 1 August 1614, conquered the town.
So feared was his pirate fleet that Spain offered Mainwaring a pardon and high command in return for his services under the Spanish flag.
According to the Venetian ambassador, Piero Contarini:He was unsurpassed in nautical skill for fighting his ship, for his mode of boarding and resisting the enemy...For nautical experience and for sea-fights, and for a multitude of daring feats performed afloat, he is in high repute, being considered resolute and courageous and perfectly suited to that profession, understanding the managing of first-rates better perhaps than anyone.