On 31 December 1831 he was charged with forging Judge Dowling's signature to a cheque on the Bank of Australia; he was found guilty and sentenced to death in 1832.

En route to Norfolk Island in 1832 Knatchbull conspired with other convicts on board ship to poison the crews' and guards' food with arsenic.

[6] In the second mutiny of 1834 planned against the governor of the convict settlement and his deputy, Knatchbull escaped punishment by informing on his fellow mutineers.

While Knatchbull was on Norfolk Island, Thomas Atkins, an Independent clergyman who was sent to Norfolk Island as a chaplain in November 1836 on the recommendation of the London Missionary Society, said of Knatchbull from his personal appearance and conversation, as all traces of a gentleman had long disappeared, he exhibited no evidence that he had been in a higher social position; indeed he appeared to be in his natural place.

In one case, a Norfolk Island expiree, who held a ticket of leave, had gone into the shop of a poor widow, named Ellen Jamieson, and asked for some trifling article.

While Mrs. Jamieson was serving him, the ruffian raised a tomahawk, which he held in his hand, and clove the unfortunate woman's head in a savage manner.

The murderer, whose name was John Knatchbull, was proved to have been a wretch of the most abominable description; and though an attempt was made to set up a plea of insanity, a barrister being employed by the agent for the suppression of capital punishment, so foul a villain could not be saved from the gallows.

[11][12] John Knatchbull sentenced to hang for the murder of Ellen Jamieson: The execution to be held outside the gates of the three-year-old [Darlinghurst] gaol, was scheduled for 9a.m.

Captain John Knatchbull aged 56 years, was led out into Forbes Street, at the gaol gates on the Darlinghurst Ridge.

Wearing genteel black broadcloth, Captain Knatchbull then "ascended the fatal scaffold without trepidation or fear, and was launched into another world with a noble and fervent prayer trembling on his lips".