Thomas Grenville was born on 4 April 1719, the seventh son of the politician Richard Grenville. His older brothers, George and Richard, also became distinguished politicians. George served as Prime Minister between 1763 and 1765. Thomas was brother-in-law to William Pitt, 1st Earl of Chatham. He was politically close to both men as all were Cobham Cubs of the Patriot Whig persuasion, a grouping led by his uncle Lord Cobham. Thomas Grenville entered the Royal Navy, and had risen to the rank of lieutenant by 1740, and captain by 1742. He was given command of the 50-gun on 6 April 1742, and while sailing off Cape St. Vincent in March 1743, he captured a Spanish ship sailing from Veracruz to Cadiz with an extremely valuable cargo, valued at £120,000 or £130,000,. In a letter to his brother George, Grenville estimated his share as being probably between £30,000 and £40,000, but it does not seem to have actually amounted to more than half. He was then appointed to the 50-gun and served off the coast of Ireland.
Later career and death
Grenville was moved to the 60-gun in 1746, and was elected as MP for the naval town of Bridport on 12 December 1746. He was ordered on an independent cruise in early 1747, due of the influence of his brother George, who was at this time one of the lords of the admiralty, but at the last moment his orders were changed and Grenville was ordered to join the fleet under George Anson. The change in orders caused annoyance, though Anson wrote to George Grenville to promise that he would detach Thomas as soon as he was able. On 3 May, Anson came across a French squadron, under the command of the Marquis de la Jonquière, off Cape Finisterre and attacked them. In the Battle of Cape Finisterre, Anson won a decisive victory, but Grenville was among the casualties. He had been hit by a large splinter two hours into the engagement, which smashed his left thigh. The leg was at once amputated, but Grenville succumbed to his wounds five hours later. His body was returned to England, where George Lyttelton wrote that the joy of the victory "is palled to our family by the loss of poor Captain Grenville, one of the most promising young menin the navy, and who, had he lived, would have been an honour not to his family only, but to his country." Thomas Grenville was buried at the family seat in Wotton Underwood. His uncle, Lord Cobham, raised a column to his memory in the grounds of his house at Stowe.