Little has been recorded about Eyles' early life, but he served for much of his time in the navy under the patronage of Sir John Borlase Warren, a prominent naval commander, who arranged for Eyles to have a significant role in the Quiberon Expedition in 1795. Success in this operation led to a promotion to post-captain, and command of his own frigate, the 44-gun . Eyles continued to served under Warren in the frigate squadron patrolling the Western Approaches, often captaining Warren's ships. Eyles saw action at the Battle of Tory Island in 1798, and later became Warren's flag captain after Warren's promotion and appointment to serve in the Channel Fleet.