Fremantle was born in 1765, and joined the navy in 1777 aged just eleven aboard the frigate HMS Hussar. Profiting from family influence, active commissions in the American War of Independence and a keen sense of seamanship and aggressive tactical awareness, promotion came easily, making lieutenant on 13 March 1782 while on duty in Jamaica and being promoted to commander on 13 November 1790 in command of the sloop HMS Spitfire. Although he did not achieve fame with his service in this period, he was in a good position to profit from the mass promotions which accompanied the outbreak of the French Revolutionary War in 1793, being made a Post Captain on 16 May 1793 in the small frigate HMS Tartar. It was in this ship that he first came to Nelson's eye, when they both served at the Siege of Bastia, where Nelson lost an eye and Fremantle gained a reputation for daring action, taking his ship under the fortress's walls despite heavy fire from overhead, which had already sunk one frigate in the bay.

The following year Fremantle was commanding the frigate HMS Inconstant when he was engaged in Lord Hotham's indecisive and cautious fleet action in the Gulf of Genoa on the 14 March 1795. The French fleet had departed Toulon and were making for the Italian coast, being chased by Hotham's fleet and an approaching storm. Fremantle, despite unspoken rules of engagement which did not require him to engage ships larger than his own, used his superior speed to overtake the 80 gun Ça Ira, which had been damaged in a collision. By taking his ship under the massive bow of his opponent, he managed to slow her enough that the oncoming British fleet was able to capture Ça Ira and another French ship which had turned back in a rescue attempt. The first British ship to the scene was Nelson's HMS Agamemnon, and the respect between the two officers continued to grow.

Nelson requested and received Fremantle as a companion and junior officer when he was detached to Italy in 1796, and the two wreaked havoc along the Italian coastline, evacuating British and royalist civilians to Corsica when the French army invaded, capturing coastal positions and raiding shore installations, capturing the island of Elba. One of the British refugees whom Fremantle rescued from Livorno was the 18 year old Catholic Betsey Wynne, daughter of Richard Wynne (from the famous Anglo - Venetian Wynne family, acquainted with Casanova) and Camille de Royer. Fremantle was so charmed by Betsey that he married her that year, with Prince Augustus as his best man. The same year he was embroiled in an engagement with Spanish gunboats off Cadiz, again under Nelson, and the next year he was with his mentor at the disastrous Battle of Tenerife, where both officers were grievously wounded in the arm. Nelson's was amputated; Fremantle's survived, but he never regained full use of it again.

Returning home on convalescence, Fremantle used the time to hone his own theories of successful command at sea, shown by several proposals he sent to the Admiralty concerning the judgment of petty disciplinary actions on board ship. Although these were rejected out of hand, they would later be used as models when the disciplinary system was revised in the 1850s. A very popular officer, loved by his men, his contemporaries and the public alike, Fremantle did not remain at home long, and when Nelson was given command of the Channel Fleet Fremantle joined him in August 1800 as commander of the ship of the line HMS Ganges. It was in this ship that he received further accolades for his service at the Battle of Copenhagen in 1801 when he was in the thick of the action. He also dabbled in politics, standing unsuccessfully for the constituency of Sandwich in 1802 before taking it in 1806.

Sent to Ireland and then Ferrol in 1803 and 1804, Fremantle was given the massive 98 - gun HMS Neptune in May 1805 and was attached to the Cadiz blockade, ready for Nelson's assumption of command later that year. At the Battle of Trafalgar that October, Neptune was third in Nelson's division, cutting the Combined Fleet shortly after HMS Victory did, and ploughing past the wrecked Bucentaure he engaged the massive Santissima Trinidad with which he endured a savage slogging match which left Neptune with 44 casualties and the outnumbered Spanish ship with over 300. Relatively undamaged, Neptune was able to tow the shattered Victory back to Gibraltar and Fremantle profited by taking the chapel silver from the big Spanish ship which he used to adorn his home.

Fremantle spent the next five years in England, serving as a Member of Parliament for Sandwich 1806 – 1807 and as a Lord of the Admiralty (1806 – 1807), before being posted rear - admiral and taking command in the Adriatic Sea, where he employed the frigate squadrons under him to great effect against French held Italy and Dalmatia. When the French empire surrendered in 1814, the entire Balkan coast surrendered to him with over 800 ships, netting Fremantle a vast fortune. For his services he was made a Knight Grand Cross of the Order of the Bath on 12 April 1815, as well as a baron of the Austrian Empire and later a vice - admiral and, from 1818, the Commander - in - Chief, Mediterranean Fleet. He also received several Austrian and Italian knighthoods as well as initiation into the Royal Guelphic Order of Hanover. Fremantle died in December 1819 from a sudden illness and was buried at Naples where his grave can still be seen in the Garden of Don Carlo Califano outside the gate of San Gennaro, Naples.

His eldest son of the same name was a famous politician, originally given a baronetcy at his father's death before later being made Baron Cottesloe for his own services to the country. Another son Charles Howe Fremantle became the Captain of the 26 gun frigate HMS Challenger the first ship to arrive in a fleet of 3 ships sent out from Britain to establish a colony at the Swan River in Western Australia. The Australian City of Fremantle is named after him. Another son, William Robert Fremantle (c.1808 - 1895) was the Dean of Ripon. His fifth son, Stephen Grenville Fremantle (1810 - 1860), was Captain in command of HMS Juno from 1853 to 1858.