From 1562 to 1805 over 3.3 million Africans were removed from their home and transported against their will to the British colonies around the world, especially the West Indies. Nelson believed that the whole Atlantic system and the success of the British Empire relied on the slave trade in the Atlantic. At the time of his naval career, the Navy relied on the trade for the training and recruitment of sailors, many of whom served in war.

Horatio Nelson was born on 29 September 1758 in a rectory in Burnham Thorpe, Norfolk, England, the sixth of eleven children of the Reverend Edmund Nelson and his wife Catherine Suckling. He was named after his godfather Horatio Walpole (1723–1809) then 2nd Baron Walpole, of Wolterton. His mother, who died on 26 December 1767, when he was nine years old, was a great-niece of Robert Walpole, 1st Earl of Orford, the de facto first Prime Minister of Great Britain. She lived in the village of Barsham, Suffolk, and married the Reverend Edmund Nelson at Beccles church, Suffolk, in 1749. Nelson's aunt, Alice Nelson was the wife of Reverend Robert Rolfe, Rector of Hilborough, Norfolk and grandmother of Sir Robert Monsey Rolfe. Rolfe twice served as Lord High Chancellor of Great Britain.

HMS Raisonnable had been commissioned during a period of tension with Spain, but when this passed, Suckling was transferred to the Nore guardship HMS Triumph and Nelson was dispatched to serve aboard the West Indiamen Mary Ann of the merchant shipping firm of Hibbert, Purrier and Horton, in order to gain experience at sea; he sailed from Medway, Kent, on 25 July 1771 sailing to Jamaica and Tobago, returning to Plymouth on 7 July 1772. He twice crossed the Atlantic, before returning to serve under his uncle as the commander of Suckling's longboat, which carried men and dispatches to and from the shore. Nelson then learned of a planned expedition under the command of Constantine Phipps, intended to survey a passage in the Arctic by which it was hoped that India could be reached: the fabled North West Passage. At his nephew's request, Suckling arranged for Nelson to join the expedition as coxswain to Commander Lutwidge aboard the converted bomb vessel HMS Carcass. The expedition reached within ten degrees of the North Pole, but, unable to find a way through the dense ice floes, was forced to turn back. By 1800 Lutwidge began to circulate a story that while the ship had been trapped in the ice, Nelson had seen and pursued a polar bear, before being ordered to return to the ship. Lutwidge's later version, in 1809, reported that Nelson and a companion had given chase to the bear, but on being questioned why, replied that "I wished, Sir, to get the skin for my father."

Nelson sailed for the East Indies on 19 November 1773 and arrived at the British outpost at Madras on 25 May 1774. Nelson and Seahorse spent the rest of the year cruising off the coast and escorting merchantmen. With the outbreak of the First Anglo-Maratha War, the British fleet operated in support of the East India Company and in early 1775 Seahorse was dispatched to carry a cargo of the company's money to Bombay. On 19 February, two of Hyder Ali's ketches attacked Seahorse, which drove them off after a brief exchange of fire. This was Nelson's first experience of battle. The rest of the year he spent escorting convoys, during which he continued to develop his navigation and ship handling skills. In early 1776 Nelson contracted malaria and became seriously ill. He was discharged from Seahorse on 14 March and returned to England aboard HMS Dolphin. Nelson spent the six-month voyage recuperating and had almost recovered by the time he arrived in Britain in September 1776. His patron, Suckling, had risen to the post of Comptroller of the Navy in 1775, and used his influence to help Nelson gain further promotion. Nelson was appointed acting lieutenant aboard HMS Worcester, which was about to sail to Gibraltar.

Worcester, under the command of Captain Mark Robinson, sailed as a convoy escort on 3 December and returned with another convoy in April 1777. Nelson then travelled to London to take his lieutenant's examination on 9 April; his examining board consisted of Captains John Campbell, Abraham North, and his uncle, Maurice Suckling. Nelson passed, and the next day received his commission and an appointment to HMS Lowestoffe, which was preparing to sail to Jamaica under Captain william Locker. She sailed on 16 May, arrived on 19 July, and after reprovisioning, carried out several cruises in Caribbean waters. After the outbreak of the American War of Independence Lowestoffe took several prizes, one of which was taken into Navy Service as the tender Little Lucy. Nelson asked for and was given command of her, and took her on two cruises of his own. As well as giving him his first taste of command, it gave Nelson the opportunity to explore his fledgling interest in science. During his first cruise, Nelson led an expeditionary party to the Caicos Islands, where he made detailed notes of the wildlife and in particular a bird – now believed to be the white-necked jacobin. Locker, impressed by Nelson's abilities, recommended him to the new commander-in-chief at Jamaica, Sir Peter Parker. Parker duly took Nelson onto his flagship, HMS Bristol. The entry of the French into the war, in support of the Americans, meant further targets for Parker's fleet and it took many prizes towards the end of 1778, which brought Nelson an estimated £400 in prize money. Parker appointed him as Master and Commander of the brig HMS Badger on 8 December.

Nelson was born into a moderately prosperous Norfolk family and joined the navy through the influence of his uncle, Maurice Suckling, a high-ranking naval officer himself. He rose rapidly through the ranks and served with leading naval commanders of the period before obtaining his own command in 1778. He developed a reputation in the Service through his personal valour and firm grasp of tactics but suffered periods of illness and unemployment after the end of the American War of Independence. The outbreak of the French Revolutionary Wars allowed Nelson to return to Service, where he was particularly active in the Mediterranean. He fought in several minor engagements off Toulon and was important in the capture of Corsica and subsequent diplomatic duties with the Italian states. In 1797, he distinguished himself while in command of HMS Captain at the Battle of Cape St Vincent.

Hinchinbrook sailed from Port Royal on 5 October 1779 and, in company with other British ships, proceeded to capture a number of American prizes. On his return to Jamaica in December, Nelson began to be troubled by a recurrent attack of malaria, but remained in the West Indies in order to take part in Major-General John Dalling's attempt to capture the Spanish colonies in Central America, including an assault on the Fortress of the Immaculate Conception, also called Castillo Viejo, on the San Juan River in Nicaragua. Hinchinbrook sailed from Jamaica in February 1780, as an escort for Dalling's invasion force. After sailing up the mouth of the San Juan River, Nelson, with some one thousand men and four small four-pounder cannon, obtained the surrender of Castillo Viejo and its 160 Spanish defenders after a two-week siege. The British blew up the fort when they evacuated six months later after suffering many deaths due to disease and Nelson was praised for his efforts. Parker recalled Nelson and gave him command of the 44-gun frigate HMS Janus. Nelson had however fallen seriously ill in the jungles of Costa Rica, probably from a recurrence of malaria, and was unable to take command. During his time of convalescence he was nursed by a black "doctoress" named Cubah Cornwallis, the mistress of a fellow captain, william Cornwallis. He was discharged in August and returned to Britain aboard HMS Lion, arriving in late November. Nelson gradually recovered over several months, and soon began agitating for a command. He was appointed to the frigate HMS Albemarle on 15 August 1781.

Nelson received orders on 23 October 1781 to take the newly refitted Albemarle to sea. He was instructed to collect an inbound convoy of the Russia Company at Elsinore, and escort them back to Britain. For this operation, the Admiralty placed the frigates HMS Argo and HMS Enterprise under his command. Nelson successfully organised the convoy and escorted it into British waters. He then left the convoy to return to port, but severe storms hampered him. Gales almost wrecked Albemarle as she was a poorly designed ship and an earlier accident had left her damaged, but Nelson eventually brought her into Portsmouth in February 1782. There the Admiralty ordered him to fit Albemarle for sea and join the escort for a convoy collecting at Cork in Ireland to sail for Quebec in Canada. Nelson arrived off Newfoundland with the convoy in late May, then detached on a cruise to hunt American privateers. Nelson was generally unsuccessful; he succeeded only in retaking several captured British merchant ships and capturing a number of small fishing boats and assorted craft.

Nelson visited France in late 1783, stayed with acquaintances at Saint-Omer, and briefly attempted to learn French. He returned to England in January 1784, and attended court as part of Lord Hood's entourage. Influenced by the factional politics of the time, he contemplated standing for Parliament as a supporter of william Pitt, but was unable to find a seat.

In 1784 he received command of the frigate HMS Boreas with the assignment to enforce the Navigation Acts in the vicinity of Antigua. The Acts were unpopular with both the Americans and the colonies. Nelson served on the station under Admiral Sir Richard Hughes, and often came into conflict with his superior officer over their differing interpretation of the Acts. The captains of the American vessels Nelson had seized sued him for illegal seizure. Because the merchants of the nearby island of Nevis supported the American claim, Nelson was in peril of imprisonment; he remained sequestered on Boreas for eight months, until the courts ruled in his favour.

Nelson spent a significant part of his career in the Caribbean. His Future wife, Fanny Nisbet, was the daughter of a slave owner on the Caribbean island of Nevis in 1787. He wrote to his Uncle, describing the £60,000 worth “stock of Negroes” and “cattle” owned by her family.

Nelson remained with Boreas until she was paid off in November that year. He and Fanny then divided their time between Bath and London, occasionally visiting Nelson's relations in Norfolk. In 1788, they settled at Nelson's childhood home at Burnham Thorpe. Now in reserve on half pay, he attempted to persuade the Admiralty and other senior figures he was acquainted with, such as Hood, to provide him with a command. He was unsuccessful as there were too few ships in the peacetime navy and Hood did not intercede on his behalf. Nelson spent his time trying to find employment for former crew members, attending to family affairs, and cajoling contacts in the navy for a posting. In 1792 the French revolutionary government annexed the Austrian Netherlands (modern Belgium), which were traditionally preserved as a buffer state. The Admiralty recalled Nelson to Service and gave him command of the 64-gun HMS Agamemnon in January 1793. On 1 February France declared war.

Early on the morning of 22 October 1793, Agamemnon sighted five sails. Nelson closed with them, and discovered they were a French squadron. He promptly gave chase, firing on the 40-gun Melpomene. During the Action of 22 October 1793 he inflicted considerable damage but the remaining French ships turned to join the battle and, realising he was outnumbered, Nelson withdrew and continued to Cagliari, arriving on 24 October. After making repairs, Nelson and Agamemnon sailed again on 26 October, bound for Tunis with a squadron under Commodore Robert Linzee. On his arrival, Nelson was given command of a small squadron consisting of Agamemnon, three frigates and a sloop, and ordered to blockade the French garrison on Corsica. The fall of Toulon at the end of December 1793 severely damaged British fortunes in the Mediterranean. Hood had failed to make adequate provision for a withdrawal and 18 French ships-of-the-line fell into republican hands. Nelson's mission to Corsica took on added significance, as it could provide the British a naval base close to the French coast. Hood therefore reinforced Nelson with extra ships during January 1794.

After the occupation of Corsica, Hood ordered Nelson to open diplomatic relations with the city-state of Genoa, a strategically important potential ally. Soon afterwards, Hood returned to England and was succeeded by Admiral william Hotham as commander-in-chief in the Mediterranean. Nelson put into Leghorn, and while Agamemnon underwent repairs, met with other naval officers at the port and entertained a brief affair with a local woman, Adelaide Correglia. Hotham arrived with the rest of the fleet in December; Nelson and Agamemnon sailed on a number of cruises with them in late 1794 and early 1795.

Nelson and the fleet remained in the Mediterranean throughout the summer of 1795. On 4 July Agamemnon sailed from St Fiorenzo with a small force of frigates and sloops, bound for Genoa. On 6 July Nelson ran into the French fleet and found himself pursued by several much larger ships-of-the-line. He retreated to St Fiorenzo, arriving just ahead of the pursuing French, who broke off as Nelson's signal guns alerted the British fleet in the harbour. Hotham pursued the French to the Hyères Islands, but failed to bring them to a decisive action. A number of small engagements were fought but to Nelson's dismay, he saw little action.

In January 1796 the position of commander-in-chief of the fleet in the Mediterranean passed to Sir John Jervis, who appointed Nelson to exercise independent command over the ships blockading the French coast as a commodore. Nelson spent the first half of the year conducting operations to frustrate French advances and bolster Britain's Italian allies. Despite some minor successes in intercepting small French warships (e.g., in the action of 31 May 1796, when Nelson's squadron captured a convoy of seven small vessels), Nelson began to feel the British presence on the Italian peninsula was rapidly becoming useless. In June the Agamemnon was sent back to Britain for repairs, and Nelson was appointed to the 74-gun HMS Captain. In the same month, the French thrust towards Leghorn and were certain to capture the city. Nelson hurried there to oversee the evacuation of British nationals and transported them to Corsica, after which Jervis ordered him to blockade the newly captured French port. In July he oversaw the occupation of Elba, but by September the Genoese had broken their neutrality to declare in favour of the French. By October, the Genoese position and the continued French advances led the British to decide that the Mediterranean fleet could no longer be supplied; they ordered it to be evacuated to Gibraltar. Nelson helped oversee the withdrawal from Corsica, and by December 1796 was aboard the frigate HMS Minerve, covering the evacuation of the garrison at Elba. He then sailed for Gibraltar.

Arms were originally granted and confirmed on 20 October 1797. The original Nelson family arms were altered to accommodate his naval victories. After the Battle of Cape St Vincent in 1797, Nelson was dubbed a Knight of the Bath and granted heraldic supporters of a Sailor and a lion.

In honour of the Battle of the Nile in 1798, the Crown granted him an augmentation of arms that may be blazoned "on a chief wavy argent a palm tree between a disabled ship and a ruinous battery all issuant from waves of the sea all proper", the motto Palmam qui meruit ferat ("let him who has earned it, bear the palm", Latin), and added to his supporters a palm branch in the hand of the Sailor and the paw of the lion, and a "tri-colored flag and staff in the mouth of the latter".

In July 1799, Nelson was created Duke of Bronté (Italian: Duca di Bronte), of the Kingdom of Sicily (after 1816, existing in the nobility of the Kingdom of the Two Sicilies), by King Ferdinand, and after briefly experimenting with the signature "Brontë Nelson of the Nile", he signed himself "Nelson & Brontë" for the rest of his life. Nelson had no legitimate children; his daughter, Horatia, subsequently married the Reverend Philip Ward, with whom she had ten children before her death in 1881.

The recall of Sir william Hamilton to Britain was a further incentive for Nelson to return, although he and the Hamiltons initially sailed from Naples on a brief cruise around Malta aboard the Foudroyant in April 1800. It was on this voyage that Horatio and Emma's illegitimate daughter Horatia was probably conceived. After the cruise, Nelson conveyed the Queen of Naples and her suite to Leghorn. On his arrival, Nelson shifted his flag to HMS Alexander, but again disobeyed Keith's orders by refusing to join the main fleet. Keith came to Leghorn in person to demand an explanation, and refused to be moved by the Queen's pleas to allow her to be conveyed in a British ship. In the face of Keith's demands, Nelson reluctantly struck his flag and bowed to Emma Hamilton's request to return to England over land.

In France, Napoleon was massing forces to invade Great Britain. After a brief spell in London, where he again visited the Hamiltons, Nelson was placed in charge of defending the English Channel to prevent the invasion. He spent the summer reconnoitring the French coast, but apart from a failed attack on Boulogne in August, saw little action. On 22 October 1801 the Peace of Amiens was signed between the British and the French, and Nelson – in poor health again – retired to Britain where he stayed with Sir william and Lady Hamilton. On 30 October Nelson spoke in support of the Addington government in the House of Lords, and afterwards made regular visits to attend sessions. The three embarked on a tour of England and Wales, visiting Birmingham, Warwick, Gloucester, Swansea, Monmouth and numerous other towns and villages. Nelson often found himself received as a hero and was the centre of celebrations and events held in his honour. In 1802, Nelson bought Merton Place, a country estate in Merton, Surrey (now south-west London) where he lived briefly with the Hamiltons until William's death in April 1803. The following month, war broke out again and Nelson prepared to return to sea.

He was a Colonel of the Royal Marines and voted a Freeman of Bath, Salisbury, Exeter, Plymouth, Monmouth, Sandwich, Oxford, Hereford, and Worcester. The University of Oxford, in full Congregation, bestowed the honorary degree of Doctor of Civil Law upon Nelson in 1802.

Nelson was appointed commander-in-chief of the Mediterranean Fleet and given the first-rate HMS Victory as his flagship. He joined her at Portsmouth, where he received orders to sail to Malta and take command of a squadron there before joining the blockade of Toulon. Nelson arrived off Toulon in July 1803 and spent the next year and a half enforcing the blockade. He was promoted to Vice Admiral of the White while still at sea, on 23 April 1804. In January 1805 the French fleet, under Admiral Pierre-Charles Villeneuve, escaped Toulon and eluded the blockading British. Nelson set off in pursuit but after searching the eastern Mediterranean he learned that the French had been blown back into Toulon. Villeneuve managed to break out a second time in April, and this time succeeded in passing through the Strait of Gibraltar and into the Atlantic, bound for the West Indies.

He was close friends with Simon Taylor, a powerful Jamaican slave owner. In 1805, responding to a request from Taylor to intervene in the public debate against abolition of the transatlantic slave trade, Nelson responded that he would fight any plans to abolish the British slave trade in England, stating “while I have an arm to fight in their defence or a tongue to launch my voice against the damned and cursed doctrine of Wilberforce”, he would support the slave trade, which he believed was essential to the success of the British Empire.

A number of monuments and memorials were constructed across the country, and abroad, to honour his memory and achievements. Dublin's monument to Nelson, Nelson's Pillar, completed in 1809, was destroyed by Irish republicans in 1966. In Montreal, a statue was started in 1808 and completed in 1809. Others followed around the world, with London's Trafalgar Square being created in his memory in 1835 and the centrepiece, Nelson's Column, finished in 1843. A Royal Society of Arts blue plaque was unveiled in 1876 to commemorate Nelson at 147 New Bond Street.

Nelson's influence continued long after his death, and saw periodic revivals of interest, especially during times of crisis in Britain. In the 1860s Poet Laureate Alfred Tennyson appealed to the image and tradition of Nelson, in order to oppose the defence cuts being made by Prime Minister william Ewart Gladstone. First Sea Lord Jackie Fisher was a keen exponent of Nelson during the early years of the twentieth century, and often emphasised his legacy during his period of naval reform. Winston Churchill also found Nelson to be a source of inspiration during the Second World War. Nelson has been frequently depicted in art and literature; he appeared in paintings by Benjamin West and Arthur william Devis, and in books and biographies by John McArthur, James Stanier Clarke and Robert Southey. Nelson is also celebrated and commemorated in numerous songs, written both during his life and following his death. Nelson's victory in the Battle of the Nile is commemorated in "The Battle of the Nile : a favorite patriotic song." Thomas Attwood's "Nelson's Tomb : a Favourite Song" commemorates Nelson's death in the Battle of Trafalgar.

Nelson was regarded as a highly effective leader, and someone who was able to sympathise with the needs of his men. He based his command on love rather than authority, inspiring both his superiors and his subordinates with his considerable courage, commitment and charisma, dubbed "the Nelson touch". Nelson combined this talent with an adept grasp of strategy and politics, making him a highly successful naval commander. Nelson's personality was complex, often characterised by a Desire to be noticed, both by his superiors, and the public. He was easily flattered by praise, and dismayed when he felt he was not given sufficient credit for his actions. This led him to take risks, and to enthusiastically publicise his resultant successes. Nelson was also highly confident in his abilities, determined and able to make important decisions. His active career meant that he was considerably experienced in combat, and was a shrewd judge of his opponents, able to identify and exploit his enemies' weaknesses. He was often prone to insecurities however, as well as violent mood swings, and was extremely vain: he loved to receive decorations, tributes and praise. Despite his personality, he remained a highly professional leader and was driven all his life by a strong sense of duty. Nelson's fame reached new heights after his death, and he came to be regarded as one of Britain's greatest military heroes, ranked alongside the Duke of Marlborough and the Duke of Wellington. In the BBC's 100 Greatest Britons programme in 2002, Nelson was voted the ninth greatest Briton of all time.

Victory came under fire, initially passing wide, but then with greater accuracy as the distances decreased. A cannonball struck and killed Nelson's secretary, John Scott, nearly cutting him in two. Hardy's clerk took over, but he too was almost immediately killed. Victory’s wheel was shot away, and another cannonball cut down eight marines. Hardy, standing next to Nelson on the quarterdeck, had his shoe buckle dented by a splinter. Nelson observed, "This is too warm work to last long." Victory had by now reached the enemy line, and Hardy asked Nelson which ship to engage first. Nelson told him to take his pick, and Hardy moved Victory across the stern of the 80-gun French flagship Bucentaure. Victory then came under fire from the 74-gun Redoutable, lying off Bucentaure’s stern, and the 130-gun Santísima Trinidad. As sharpshooters from the enemy ships fired onto Victory’s deck from their rigging, Nelson and Hardy continued to walk about, directing and giving orders.