Midshipman on board the Bounty.

A loyalist.

Hallett was only 15 years old when he mustered on the Bounty, which sailed from Spithead on 23 December 1787.

In many ways, he and Thomas Hayward were like peas in a pod. At just 15 years of age, both were the youngest on board. Both had come on board by the influence of Ms. Bligh who knew their respective families well (Ann Hallett, John's sister, seems to have been a bosom friend of Betsy Bligh). Both were protégés of botanist Joseph Banks. Both were disliked on the ship because of snobbishness and arrogance, both had a tendency to sleep on duty, and when the mutiny broke out, both had tearfully begged to be allowed to stay on board the Bounty while other loyalists were going into the launch. Yet both later testified against those loyalists who had been forced to stay on board the ship.

In April 1789 Fletcher Christian in the mutiny forced Hallett to accompany Captain William Bligh on his open boat voyage.

When the mutiny broke out, Hallett had not even appeared on deck even though he was in Christian's watch. His – and Hayward's – disinclination to do any work clearly made it easier for Christian to take over the ship.

Hallett's testimony during the court-martial of the accused mutineers was highly damaging to Heywood. Hallett claimed that he had observed Bligh saying something to Heywood during the mutiny and the latter had just laughed and turned around and walked away. After the publication of Edward Christian's Appendix, Bligh needed help in defending himself against the charges that had been leveled at him, and he found a willing instrument in Hallett who claimed that, no matter what anyone else had said, Bligh had never accused anyone of stealing any coconuts.

Hallett later became a lieutenant and died on board HMS Penelope.