In 1787 he attained flag rank, and in 1788 he married his cousin Martha Parker. With the outbreak of war with Revolutionary France (note: at this point France was not under Napoleon as Napoleon's coup did not take place until October of 1799) he was put into service in the West Indies co-operating with the Army in the conquest of the French islands. On return to Great Britain in 1795 he was promoted to Admiral. In November he took command in the Mediterranean, where he maintained the blockade of Toulon, and aided the allies of Great Britain in Italy.
He was Commander-in-Chief of the British Mediterranean Fleet from 1796 to 1799. One of his chief duties was to watch the Spanish fleet at Cadiz. He defeated the Spanish fleet at the Battle of Cape St Vincent. For this victory he was created Baron Jervis, of Meaford in the County of Stafford, and Earl of St Vincent.
That same year, however, the Spithead and Nore mutinies threatened the Navy. He prevented any outbreak in his command through foresight and severity, including the flogging and hanging of sailors and public berating of his officers, one of whom, Sir John Orde challenged him to a duel. Nevertheless, he raised the discipline of the Navy to a higher level than it had reached before; he was always ready to promote good officers, and the efficiency of the squadron with which Lord Nelson won the Battle of the Nile was largely due to him.
Lord St Vincent resigned his command temporarily in 1799, due to his failing health, but he recovered and resumed command the following year, and became First Lord of the Admiralty in 1801, also being created Viscount St Vincent, of Meaford in the County of Stafford, with a special remainder. He took command of the Channel Fleet from 1806 to 1807, and retired from the Navy in 1811. In 1821 he was promoted to Admiral of the Fleet on the occasion of the coronation of King George IV.
When he died in 1823, the Barony of Jervis and the Earldom of St Vincent became extinct, but the Viscountcy of St Vincent passed to his nephew.
There is a monument to him in St. Paul's Cathedral, and portraits of him at different periods of his life are numerous.
"I do not say, my Lords, that the French will not come. I say only they will not come by sea." addressing the House of Lords as First Lord of the Admiralty in 1801.
