In 1766 he entered the British Army as an Ensign in the 50th Foot. He was promoted to Lieutenant in 1772, and stationed at Jamaica, where he soon proved himself to have considerable engineering talent.
When the American War of Independence broke out the regiment, significantly under strength, remained in the West Indies rather than joining the army of William Howe attempting to suppress the rebellion. He served in the West Indies with credit, being promoted to Captain after taking part in the unsuccessful 1780 San Juan expedition. Despard struck up a friendship with the naval officer Horatio Nelson, who played a leading role in the San Juan campaign. In 1782 he commanded a successful expedition to recover the British settlement of Black River on the Mosquito Coast of present-day Honduras, which the Spanish had taken. In 1783 the war was brought to an end by the Peace of Paris.
Honduras
Despard was subsequently made Superintendent of the Bay of Honduras, which later became British Honduras and then Belize. He administered this British enclave until 1790 when he had married a young black woman, Catherine Despard, and staked his reputation on giving the same rights to freed slaves as to white settlers. This, however, did not go down well with some of the settlers. These settlers sent letters of protest to London, and as a result Despard was summoned back to London to explain himself. He was suspended by Home SecretaryLord Grenville. From 1790 to 1792 these charges were investigated, and he was suspended on half pay with his expenses from the Bay of Honduras withheld. Pursued by a further lawsuit from his enemies in the Bay, he was arrested and confined in King's Bench Prison from 1792 to 1794.
Radical
On his release he joined the London Corresponding Society. In 1798 Despard was arrested on suspicion of involvement in the Irish Rebellion. Habeas Corpus had been suspended in 1794, and Despard was held without trial for nearly three years in a succession of prisons, notably Coldbath Fields Prison in Clerkenwell, until he was released without charge in 1801.
Despard Plot
In late 1802 he was named by government informers and disaffected soldiers as a member of a conspiracy engaged in a plot to seize the Tower of London and Bank of England and assassinate King George III to encourage further uprisings. On 16 November 1802, Colonel Edward Marcus Despard and his co-conspirators were arrested at the Oakley Arms public house at 72 Oakley Street Lambeth on suspicion of plotting the uprising. The evidence was thin but in all, some forty people were arrested and they all appeared before magistrates at Union Hallpolice office the following day. Their conspiracy had been betrayed by one of the group, Thomas Windsor, who was the chief witness at their trial. They were prosecuted by Attorney GeneralSpencer Perceval, before Lord Ellenborough, the Lord Chief Justice. Despite a dramatic appearance by Lord Nelson as character witness on his behalf, Despard and his co-conspirators were tried before a Special Commission on Monday, 7 February 1803, for conspiring to capture and kill the King and overthrow the government and for planning to stop the mail coaches entering and leaving London and the take over of the Tower of London. Despard was found guilty by the jury of 3 counts of high treason along with John Wood, 36, John Francis, 23, both privates in the army, Thomas Broughton, 26, a carpenter, James Sedgwick Wratton, 35, a shoemaker, Arthur Graham, 53, a slater, John Macnamara, Thomas Newman, Daniel Tindall, and William Lander. All but Newman, Tindall and Lander who were respited and later transported as convicts to Australia were to be hanged, drawn and quartered. Prior to execution the sentence was commuted to simple hanging and beheading, amid fears that the draconian punishment might spark public dissent. Despard was executed on the roof of the gatehouse at Horsemonger Lane Gaol, in front of a crowd of at least 20,000 spectators, on 21 February 1803. This was the largest public gathering until the funeral of Lord Nelson following the Battle of Trafalgar.
