By 1757, and aged 15, Harvey was certainly at sea, making junior lieutenant aboard the fourth-rate HMS Hampshire in the English Channel, the West Indies and along the North American coast during the Seven Years' War.

During the voyage home on parole aboard HMS Dragon, Harvey made close friends with Lieutenant Constantine Phipps, who later became a Lord of the Admiralty.

In 1773 an opportunity for adventurous service arrived with the offer of a position on Phipps's ship HMS Racehorse in which Harvey's friend was planning to explore the North Pole.

With a promotion resulting from the expedition enabling further advancement, Harvey commanded the sloop HMS Martin at the Siege of Quebec in the American Revolutionary War of 1776.

His knowledge of the St. Lawrence River and Canadian coastline gave him an advantage in this work and as a result he was raised in 1777 to command the frigate HMS Squirrel on convoy duty.

December 1778 saw a transfer to the 32-gun frigate HMS Convert and in the following year Harvey was engaged in the relief of besieged Jersey and later unsuccessful efforts to intercept the raiding squadron of John Paul Jones.

[1] Within weeks, Harvey had been transferred to HMS Rose and, aboard her, joined Pegasus in peacetime manoeuvres off the North American station until the ship was paid-off in 1789.

Ramillies was present with Admiral Lord Howe's fleet at the battle of the Glorious First of June in 1794 and in her Harvey distinguished himself by rescuing the stricken HMS Brunswick commanded by his brother John.

[1] By 1799 Harvey was contemplating retirement and passed the station to Lord Hugh Seymour, returning to England and raising his flag in HMS Royal Sovereign as second-in-command of the Channel Fleet until the Peace of Amiens in 1801.