Thomas entered the navy at a very early age, and served during the Seven Years' War with his uncle Samuel on board HMS Scorpion, Duke, and Venus.

In the following year he went out to North America with his uncle Samuel, and was appointed by him to command HMS Diana, one of the small schooners employed for the prevention of smuggling.

[1] In the following autumn Graves was appointed to the frigate HMS Magicienne, in which, on 2 January 1783, he fought a very severe action with the French Sibylle, which was encumbered with a second ship's company which she was carrying to the Chesapeake.

Both frigates were reduced to a wreck, and so parted; Magicienne to get to Jamaica a fortnight later; Sybille to be captured on 22 January 1783 by Hussar under Thomas McNamara Russell.

This was only for a few months; for on 1 January 1801 he was promoted to be Rear-Admiral of the White Squadron, and in March hoisted his flag on board the 64-gun HMS Polyphemus, one of the fleet proceeding to the Baltic with Sir Hyde Parker.

Towards the end of July the fleet left the Baltic, and on its return to England Graves, who had been in very bad health during the greater part of the campaign, retired from active service.