She played a major part in the 1757 Rochefort expedition, helping to silence the batteries on the Isle of Aix, and served at the Battle of Quiberon Bay in 1759 under Lord Howe, where she forced the surrender of the French 74-gun Héros.

Le Magnanime was built between 1741 and 1745 in the port of Rochefort on the Charente estuary, France, and was designed by the renowned shipwright Blaise Geslain.

She served as Rear-Admiral Savage Mostyn's flagship, part of the Channel Fleet commanded by Vice Admirals Edward Boscawen and later, Charles Knowles.

[2][6] The plan to take Rochefort itself was later abandoned but Île d'Aix was captured and in the preceding battle, it was Magnanime's guns that bombarded the island's fort into submission.

[8] The island was extremely important to the port of Rochefort because ships of the line were required to load and unload supplies and armaments there, being unable to navigate the shallow river fully laden.

[9] Hawke had formed an advanced squadron, under Sir Charles Knowles, comprising Magnanime, Barfleur, Neptune, Torbay and Royal William, and at around noon he sent these ships on ahead.

[8] The squadron was now without a competent pilot, who was aboard Magnanime, and the fleet, Hawke having by this time caught up, had to wait until the three ships returned the following afternoon.

Conflans had slowed on the night of the 19th in order to arrive at Quiberon at dawn, and to investigate a small squadron of ships under Admiral Robert Duff, 20 miles (32 km) off Belleisle.

[15] Conflans was faced with the decision to stand and fight in rough seas and unfavourable winds, or to attempt to reach the hazardous waters around Quiberon Bay before nightfall.

This was a temporary command however and Hughes was replaced by Captain Charles Saxton early in 1762 where she served as flagship to Commodore J. Cerrit in the Basque Roads.