Francis John Fox was born in County Westmeath, Ireland, probably on 20 September 1857, the son of Dorothea West and her husband, Jemmett George Fox, an army officer. Educated at the Royal Military Academy, Woolwich, he was commissioned as a lieutenant in the Royal Regiment of Artillery in February 1876.
Fox served in India from 1877 to 1880, South Africa in 1881 and Egypt in 1885, taking part in the second Afghan war, the first South African war and the Egyptian campaign. Between these periods of active service, he held a series of staff appointments in England, including aide-de-camp to the inspector general of artillery (1886–89) and to the officer commanding the North Western District and later the Thames District.
When in 1891 the New Zealand government sought an imperial officer to take charge of the country's military forces, the War Office nominated Fox, by then a captain, for the post. Despite his relatively low rank, he came with impressive references. From 'an old and high class Irish family', he was, according to his former commanding officer, a man of 'tact, manners, influence derived thro' his knowledge of his work', and gifted with 'extraordinary cleverness' and power of 'personal magnetism'. He arrived in New Zealand in May 1892 to take up a five-year contract as commandant of the New Zealand forces with the rank of lieutenant colonel.
Francis Fox's first step was to secure permission from the minister of defence, Richard Seddon, to report on the colony's defence resources. This took much longer than Seddon had anticipated, for Fox was determined to inspect every corps of the Volunteer Force. His report, which appeared in three instalments in 1893, caused a sensation because of its uncompromising nature, especially in its comments on the fitness of officers for their commands. The approximately 25 per cent deemed indifferent or bad soon became known as 'Fox's Martyrs'. Many overseas, and indeed local, officers had noted the deficiencies of the Volunteer Force in the past, but Fox became embroiled in a simmering dispute with Seddon over what should be done to remedy them. He sought specific actions, some of which would be politically difficult for Seddon, by now installed – not completely securely – as premier. Fox also sought extensive powers as commandant, akin to those enjoyed by his counterparts in other colonies.
Fox's approach was counter-productive. He had begun, as a Colonial Office official noted, 'by putting a pistol at the Minister's head, and dictating to him in an absurd manner', and Seddon not surprisingly had reacted unfavourably. The deteriorating relationship between the two reached breaking point in March 1894 when Fox sought a release from his contract and a return fare to Britain. His letter of resignation, and an accompanying catalogue of complaints about Seddon's response to his recommendations, was leaked to the public by the Evening Post, causing a furore. Seddon was probably right to suspect political machinations, for Fox was on close terms with the governor, Lord Glasgow, who angered Seddon by intervening privately in support of him, and with the leader of the opposition, W. R. Russell: Fox married Russell's daughter, Cara Beatrice, at Hastings on 6 February 1895.
Although angered by Fox's conduct, Seddon did not accept his resignation. But it was recognised soon afterwards that his appointment was ultra vires: there was only statutory provision for the appointment of a commander, not a commandant, of the New Zealand forces. After some negotiation Fox accepted appointment, at a lower salary, as military adviser to the New Zealand government and inspector of the New Zealand forces. In this capacity he was able to make some progress in reforming the colony's military forces. His willingness to fight for improvements in the face of an unsympathetic government was appreciated by many volunteers.
By the time Francis Fox's service came to an end, on 30 November 1896, he had revised his intention to return to Britain. In May 1894 he had retired from the imperial army. In partnership with his brother, Wilder B. Fox, he had acquired a 1,300-acre farm, Te Kouka, near Waikare in Canterbury. From December 1896 Fox became 'quite an enthusiastic farmer', having taken over the management of Te Kouka from his brother. Early in 1897 he bought another property of just over 6,000 acres near or adjacent to Te Kouka, using Cara Fox's money for the deposit and obtaining a worryingly large mortgage. This new property was leased to the Te Kouka partners, including Francis himself. By 1898 Birchdale, as he named the combined holding, was running more than 6,000 sheep. The following year the partnership was dissolved and Francis bought out his brother's interest. By this time he and Cara had had two sons and a daughter.
When the second South African war began in late 1899 Fox, who held the rank of colonel in the New Zealand Militia, made several attempts to obtain commissions in the New Zealand contingents; he even offered to serve as a captain. All were unsuccessful, possibly because of Seddon's antipathy towards him. Disgusted, Fox severed his last connection with the New Zealand military forces by resigning his militia commission.
During the winter of 1900 Fox's health deteriorated alarmingly. Eventually diagnosed as suffering from consumption, he went, in early 1901, to a sanatorium at Wentworth Falls in the Blue Mountains, west of Sydney, New South Wales, and was later joined there by his wife. Although he returned to New Zealand in September 1901 and settled at Bluff Hill in Napier, his condition remained serious. He died suddenly of a haemorrhage on 27 February 1902 and was buried in Napier cemetery. He was survived by Cara Fox and their three children.
Francis John Fox was the first in a succession of British officers who were to take charge of the New Zealand forces over some quarter of a century. He succeeded in remedying some of the defects of the volunteer system, although it did not long survive his tenure of office.