Jervis, Staffordshire-born son of a lawyer, was educated at Burton-on-Trent Grammar School and at Swinden's (formerly Weston's) school, Greenwich, a fellow pupil at the latter being James Wolfe, with whom he later served in the capture of Quebec. He was intended for the law but, after trying to run away to join the Navy, was allowed to do so in 1749 and was commissioned lieutenant in 1755. He earned a reputation as both a highly professional fighting captain and a disciplinarian during the Seven Years War and the War of American Independence, was knighted in 1782 and promoted rear-admiral in 1787.

A vice-admiral at the start of the French Revolutionary War, he and General Sir Charles (later Earl) Grey captured Martinique and Guadeloupe in 1794. As an admiral, he became commander-in-chief in the Mediterranean in 1795, succeeding the ill and ineffective William Hotham. A good judge of professional character, he ruthlessly weeded out captains and other officers he considered weak in favour of rising stars including Nelson, whom he promoted to commodore as soon as possible rather than lose him elsewhere. In 1796, however, because of the French advance into Italy and Spain's imminent alliance with them (from October), he had to abandon the Mediterranean itself and base the fleet at Gibraltar and off Cadiz. When the Spanish fleet sailed early in 1797 he intercepted and defeated it off Cape St Vincent in the action that first brought Nelson to public fame, and was himself created Earl of St Vincent. With Admiralty approval he was responsible for detaching Nelson to reconnoitre the Mediterranean early in 1798 and then reinforcing him with the squadron that defeated the French at the Battle of the Nile that August.

In 1799 St Vincent's health led to his return home but in 1800 he took command of the Channel Fleet, enforcing the same discipline he had required in the Mediterranean. In 1801-04 he was a First Lord of the Admiralty under the Addington ministry, in which role his Whig political convictions and obsession with rooting out largely imagined corrupt practices in the dockyards and naval administration nearly brought the Navy to logistical disaster in the pre-Trafalgar period. Fortunately he was replaced on Pitt's return to office by the 79-year-old Charles Middleton, Lord Barham, whose administrative expertise swiftly addressed the damage caused. None the less, St Vincent's investigations of the often archaic, if pragmatic, naval infrastructure of his time paved the way for later necessary reforms after 1815.

St Vincent refused further service under Pitt but following Pitt’s death in February 1806 briefly resumed command of the Channel fleet until 1807. In a long retirement he retained interest in naval affairs and was much honoured, including with the rank of Admiral of the Fleet from 1821. This was unusual in that there was normally only one such post-holder at a time and he was a second, with HRH the Duke of Clarence (William IV from 1830). St Vincent's character was one of rigorous self-sufficiency and professionalism, both acquired from the hardships of his early life, linked with often grim humour at the discomforts which he created for others but willingly endured himself, and a consistent distaste for public ostentation. He could be excoriatingly critical but was also known for much private kindness to those who had any claim on his friendship or deserving support.

The prime version of this portrait of St Vincent is in the Royal Collection, having been exhibited at the Royal Academy in 1809, the year before Hoppner's death. This copy was presented to the Naval Gallery at Greenwich Hospital by George IV in 1824 and is presumed to have been made at about that time.