John Lynch (c. 1812 – 22 April 1842) was an Irish-born Australian serial killer who confessed to the killing of ten people between 1836 and 1842. He is possibly the most prolific individual serial killer in Australian history. Lynch arrived in Australia as a convict, assigned to a farm in the Berrima district. He murdered a fellow assigned convict in 1836 but was acquitted of the charge. After a period in a convict gang he absconded, and by July 1841 he had made his way back to the Berrima district. On two occasions Lynch murdered carriers along the road between Berrima and Camden, stealing their drays and teams. In the latter half of 1841 Lynch murdered the farmer John Mulligan and his family, and took possession of their farm in the Berrima district using the name John Dunleavy. He was convicted in March 1842 of the murder of Kearns Landregan, sentenced to death and executed by hanging in April 1842.

John Lynch was born in about 1812 in Cavan, county Cavan, Ireland, the son of Owen Lynch.

Lynch's older brother Patrick was tried and convicted at Cavan in July 1831 for sheep-stealing, sentenced to transportation for life. He was transported to Sydney aboard the Captain Cook, arriving in April 1832.

John Lynch and his father Owen were both tried and convicted at Cavan in October 1831. John Lynch, aged 19 years, was convicted of “obtaining goods under false pretences”, for which he received a life sentence; his father, a widower aged 55 years, was convicted of “having stolen goods in possession”, receiving a sentence of transportation for seven years. Lynch and his father were transported to the colony of New South Wales aboard the Dunvegan Castle, which departed from Dublin on 1 July 1832 and arrived at Sydney on 16 October. Upon their arrival John Lynch, recorded as a ploughman, was assigned to James Atkinson at 'Oldbury Farm' in the Bong Bong district near Berrima. Owen Lynch was assigned to Richard Brownlow, a publican of Sydney.

Lynch's father, Owen Lynch, died on 26 February 1834 in the Gaol Hospital at Sydney, aged about 58 years.

In about January 1836 George Barton, the overseer of ‘Oldbury Farm’, was waylaid on a district road "by ruffians, supposed to be bushrangers", who robbed him, tied him to a fence and whipped him. It was suspected that the perpetrators were a gang of bushrangers operating in the area led by John Wales (alias Watt). In mid-February 1836 Watt was captured, along with another gang member Timothy Pickering, after a shoot-out with the police on the Cowpasture River, during which a third gang member (John Carpenter) was killed. In early May Watt and Pickering were tried for an armed robbery committed previous to their apprehension, found guilty and sentenced to hang. The sentence was carried out eight days later when both men were hanged. The status of George Barton, the overseer, changed in February 1836 when he became the master of ‘Oldbury Farm’ after marrying Charlotte Atkinson, widow of James Atkinson (who had died in April 1834).

On Friday, 12 August 1836, two of Barton's assigned convicts, John Lynch and John Williamson, were tried in the Sydney Supreme Court before Justice Burton for the “wilful murder” of Thomas Smith (or Smyth), another of Barton's assigned servants. Smith had been murdered on 4 March 1836; his body was discovered after several days in the hollow of a fallen tree about a mile from the convict huts at ‘Oldbury Farm’. Two heavy pieces of wood “clotted with blood and human hair” were found nearby. Barton's overseer gave evidence that, prior to his murder, Smith had been held in the Bong Bong watch-house in order to be examined by the district magistrates on a charge of having lost or stolen a saddle and bridle belonging to his master. The overseer had turned up at several court days in order to prosecute the case but on each occasion the magistrates failed to attend, after which it was decided to release Smith, in consideration of the punishment he had already undergone and the inconvenience of the loss of his services. Smith returned to his hut after his release and was found to be missing the next morning, with his body being discovered several days later.

The case against Lynch and Williamson rested primarily on the testimony of Michael Hoy, another convict at ‘Oldbury Farm’, who lived in the same hut as Smith, Lynch and Williamson. Hoy claimed that Lynch and Williamson had lured Smith from the hut the evening of his disappearance. Hoy maintained the motive for the murder was a suspicion that Smith had provided information that Lynch was one of those involved in the previous robbery and assault of Barton (this being the purported reason Smith was released). After the discovery of Smith's body, Hoy asserted he had had a conversation with Lynch, in the presence of Williamson, when Lynch was supposed to have said “that the master had come to the knowledge that he (Lynch) was one of the men that assisted Watts to flog him about three months before, which he could not have done if Smith had not told him something about it, and therefore he was glad he (Lynch) had put him out of the way”. When Hoy was cross-examined by the prisoners it was elicited that as a consequence of "some dealing in cattle" between Hoy and Smith, Hoy himself may have had reason to murder Smith. At the very least Hoy's testimony was significantly discredited. Barton, the master, was called to the stand, but his evidence "was rejected in consequence of his appearing 'to have dined'". As well as dismissing his evidence, Justice Burton fined Barton £50 for coming into the witness-box in a drunken state. In summing up the judge explained to the jury that, with the absence of corroborative evidence, the case rested solely on the credibility of the witness Hoy, which he considered to be “a person tainted with crime, and therefore liable to suspicion”. The jury returned a verdict of not guilty for both men. Six years later, on the day before he was to hang, Lynch confessed to Smith's murder.