Joseph Burns was born in Liverpool, England, in 1805 or 1806 of Irish parents. He joined the Royal Navy as a ship's carpenter at about the age of 20, and arrived at the Bay of Islands, New Zealand, on the Buffalo in 1840. On 28 July the ship was wrecked at Mercury Bay, and Burns took his discharge; he was then employed by the government. He later moved to Auckland, where his first employment was with a local boatbuilder.
Burns won a reputation for heavy drinking, which he maintained was necessary to dull the pain of severe headaches caused by head injuries suffered previously in a fall from a mast. He and Margaret Reardon, a married woman separated from her husband, lived in a shack Burns had built in Mechanics Bay. Two sons resulted from this liaison. In 1845 Burns worked in a market garden until he was dismissed for assaulting the foreman.
Settlers were reluctant to employ him but he finally found work on the North Shore as a farm labourer for James Harp. Early in 1847 Harp dismissed Burns for stealing and butchering his stock. He was evicted from the farm cottage and subsequently built a rough dwelling for his family at Shoal Bay; they survived on what Burns earned in casual labour for the chief Patuone, supplemented by vegetables grown by Margaret Reardon.
On 22 October 1847 Burns, desperate for money, murdered a naval lieutenant, Robert Snow, his wife and daughter, for the sake of £12 in naval pay kept in their house. The bodies were mutilated to suggest a Māori attack, and the house burned. Māori camped in the vicinity were suspected but at the inquest Dr John Johnson, coroner, returned a verdict of murder by person or persons unknown. The case raised fears of an imminent Māori attack on Auckland.
Margaret Reardon left Burns as a result of the murders, and took the children to her sister, Sophia Aldwell, in Shortland Crescent. Burns then joined the naval steamer Inflexible, which departed for Australia on 6 November 1847 and returned to Auckland on 11 December. On 28 December he visited Margaret Reardon to persuade her to marry him so that she could not be compelled to give evidence against him; she refused. In a drunken fury he savagely attacked her and then attempted suicide. He was arrested and at the Supreme Court criminal sessions on 1 March 1848, he was convicted of grievous bodily harm and sentenced the following day to transportation for life.
Burns was now fearful that Margaret Reardon would implicate him in the Snow murders. He asked her to visit him with the two boys and coerced her into backing up a false confession in which he accused Thomas Duder and William Oliver, his former shipmates from the Buffalo, of the crime. They were exonerated after Burns retracted. On 1 June he was charged with murder, and accused Margaret Reardon, the chief prosecution witness, of inciting him to make the false confession. She admitted perjury and recounted the true sequence of events.
Burns was pronounced guilty of wilful murder by Chief Justice William Martin and sentenced to hang. On 17 June 1848, having made a full confession, which unfairly implicated Margaret Reardon, he was taken under escort from the gaol, across the harbour to the site of his crime and hanged before a large crowd of settlers and Māori. He was the first European in the colony to be executed for a capital crime.
In September 1848 Margaret Reardon was convicted of perjury and sentenced to seven years' transportation.