Little is known of the early life of John Manning. He was educated in New York; his family had probably migrated to the United States from Ireland when he was a child. As a young man, he went to the Victorian goldfields. He was a journalist for the Ballarat Times during the Eureka Stockade incident in December 1854. Many of those involved in this revolt against colonial bureaucracy, including the leader, Peter Lalor, were Irish Catholics. Manning helped to organise a defence on military lines when it became clear that the authorities planned to quell the uprising by force. One of his fellow fighters at Eureka, Raffaello Carboni, described him as warm-hearted and impulsive, a thorough Irishman, with a great love of the bottle. Manning was in the stockade during the assault on the morning of 3 December and was arrested along with the other defenders. He waited four months in Melbourne gaol before being acquitted on a charge of high treason, as juries refused to convict the prisoners.
Manning migrated to New Zealand in 1860 and turned up in Hokitika in 1867, at his old trade of goldfields journalism. He was one of the co-founders of the New Zealand Celt, the first issue of which came out on 26 October 1867. The paper strongly supported the cause of Irish nationalism. In early 1868 word reached Hokitika of the hanging at Manchester of the three Fenians William Allen, Michael O'Brien and Michael Larkin. This news was a pretext to declaim further against the injustices suffered by the Irish under English government. Father William John Larkin, who had helped Manning to establish the New Zealand Celt, was pastor of Waimea and Staffordtown. These areas, together with Kaniere and Totara Flat, had many Irish settlers, most of whom were Catholics. Manning and Larkin helped to organise and promote a commemorative funeral procession in Hokitika. Protesters took an empty coffin and Celtic cross to the local cemetery in defiance of a ban by the Hokitika Municipal Council, whose mayor, William Shaw, was proprietor of the West Coast Times, and a bitter opponent of the New Zealand Celt.
In the midst of this controversy an Irishman and suspected Fenian, Henry James O'Farrell, attempted to assassinate the young Duke of Edinburgh in Sydney. Manning and Larkin, and five others, were arrested in a wave of loyalist feeling, much stirred up by the West Coast Times. Manning and Larkin were convicted of seditious libel at a trial held in May, and were imprisoned. After serving the nominal term of one month in gaol, Manning resumed the editorship of the New Zealand Celt. He was clearly not too mindful of the admonition of Judge C. W. Richmond to tone down the militancy of the paper. 'Irish nationality is…the feature inseparable from the CELT, and this feature shall be maintained as long as I am its editor,' he stated in an editorial in June 1868.
In spite of these brave words, by mid August a letter appeared in the paper urging its readers to support a testimonial and address for Manning, who had decided to leave the colony in consequence of the passing of the Treason Felony Act. He had departed by the end of the month for the United States. There he continued his career as a journalist, writing stories and articles on Australian and Maori themes for newspapers such as Chambers Journal of Popular Literature and Art (Edinburgh) and the Overland Monthly.