John Thomas Marryat Hornsby was born at Hobart, Tasmania, on 13 March 1857, the son of Sarah Turner and her husband, John Marryat Hornsby, a constable who later became a general printer. Hornsby junior was brought up in the printing trade by his father. He moved to Nelson, New Zealand, in 1874 with his family and stayed on when the other members returned to Hobart. After working briefly at Queenstown on the Lake Wakatip Mail, he took charge of the Arrow Observer at Arrowtown. There, on 4 February 1876, he married Sarah Scott Napier. They were to have one child, a son.
Hornsby, who was generally known as J. T. Marryat Hornsby, joined the Southland Times at Invercargill in 1876 as foreman printer. He later worked on the Southland Daily News. Hornsby wrote several plays; one, The Kellys, about the Australian outlaws, caused considerable controversy when it was performed in Invercargill in July 1880. Hornsby returned to Arrowtown in 1882 and started the Lake County Press. In 1886 he became editor of the Napier Evening News and Hawke's Bay Advertiser and later also edited the Waipawa Mail.
In 1894 Hornsby was appointed editor of the Christchurch Star. He left after barely a year following differences with the proprietors over editorial policy. In June 1895 he promoted a weekly paper, the Sun. Although the paper failed before the end of the year, it helped expose the activities of Arthur Worthington, a confidence trickster and bigamist. Hornsby became sub-editor of the New Zealand Times in Wellington in August 1896 and in December that year he became the editor.
Newspaper editors and proprietors in New Zealand frequently became involved in politics in the nineteenth century. Both a parliamentary career and newspaper editorship were largely concerned with boosting the reputation of a district. Hornsby's political career began at Arrowtown when he was a borough councillor (1874–76) and lieutenant in the volunteers. In 1884 he unsuccessfully contested the parliamentary seat of Wakatipu. While living in Hawke's Bay he was a member of the education board.
In 1896 Hornsby stood for Wairarapa as a Liberal, losing by 333 votes. His supporters petitioned, alleging bribery by the followers of Walter Buchanan, the oppositionist who won the seat. They were not successful. Hornsby moved to Carterton in 1897 to edit a tri-weekly newspaper, the Wairarapa Leader. He henceforth based his career on promoting Wairarapa small-town interests. He was to relinquish control of the Leader in 1906 when he became editor of the New Zealand Railway Review.
Hornsby won the Wairarapa seat in 1899 but lost it in 1902. At the seven elections between 1896 and 1914 Hornsby and Buchanan were usually the only candidates for Wairarapa and the seat regularly changed hands. There was a remarkably clear distinction between their political bases, with Hornsby favoured by the majority of the small towns, and the runholder, Buchanan, gaining strong support from the farming districts.
Hornsby regained the seat in 1905 but lost it in 1908 and failed again in 1911. This was something of a surprise in view of his prominence in lobbying his ministerial colleagues to buy and subdivide the local Carrington estate, a popular cause in an electorate which generally favoured closer settlement of the land. Hornsby was always strongly against land monopoly, a principal concern of many of his constituents who saw towns as being strangled economically by large surrounding estates. He won the Wairarapa seat once more, in 1914, but was defeated in 1919 by the Reform Party candidate, Alexander McLeod.
Hornsby espoused political views that were typical of the country interest at this period, but with some variations which were related to his need to maintain his small-town base in Wairarapa. He was a free trader and regularly opposed factory legislation and attempts to reinforce or expand the rights of trade unions. He initially favoured giving freehold to holders of state leases-in-perpetuity only if there were restrictions attached to the grant; for example, limiting the right to mortgage the freeholded property. He argued for checks on land aggregation and limitations on the amount of land any individual could own. In 1907 he took a more conservative line in supporting the right of a lessee to freehold land at the value when first leased without adding the value accrued to it by the lessee's own efforts.
Hornsby's relations with the Liberal party leaders were often prickly. He supported the elective executive idea by which cabinet ministers would be chosen by the House. He had studied elocution and was an eloquent and effective speaker, a good raconteur with 'a great fund of anecdote and humour'. J. T. Marryat Hornsby died at Carterton on 23 February 1921. Sarah Hornsby died in 1934.