Shortly after qualifying to practice as a lawyer, Fox married Sarah Halcomb. The couple decided that, rather than remain in England, they would immigrate to New Zealand, joining an increasing number of other colonists. Upon his arrival in Wellington, Fox's legal qualifications were recognised, but there was little work, and so he supplemented his income by writing for local periodicals. Fox lost the right to practice as a lawyer when, in 1843, he refused to swear an oath that he considered "degrading". This event forced him to focus almost entirely on writing and journalism.
In Nelson, Fox met with mixed success. There was little direct conflict with the Māori, and most of Fox's work was related to economic development. Poor planning and inaccurate land surveying had left colonists with considerably less than had been promised them, and Fox was responsible for resolving the matter. While many modern historians believe that he did a good job, Fox himself found that even his best efforts were not good enough for the angry colonists. Fox increasingly spent his time leading parties into the wilderness near Nelson, an activity which he seems to have enjoyed. Fox was physically active all through his life.
In 1848, William Wakefield (brother of Arthur, killed at Nelson), died. William Wakefield, as the New Zealand Company's senior officer in the colony, was Fox's superior. Fox quickly travelled to Wellington, and managed to secure himself in Wakefield's position. He accomplished this mainly because of the short distance between Nelson and Wellington, which enabled him to win the position before instructions could be received from other cities. He was not the first choice of the Company's board in London (which preferred Dillon Bell), but his quick action managed to gain him enough support to receive the appointment.
The Company, however, was in decline after the deaths of both Edward and Arthur Wakefield. Fox gradually became less active in the Company, taking more of an interest in the colonial government. He was a strong opponent of Governor George Grey, who was refusing to grant self-government to the settlers. He frequently denounced the administration and the judiciary as corrupt and incompetent.
In 1851, Fox travelled to London on behalf of a group of Wellington settlers. There, he met with Edward Gibbon Wakefield, elder brother of Edward and Arthur. He discussed his ideas about a constitution for New Zealand, strongly supporting self rule, provincial autonomy, and two elected houses of parliament. He also attempted to meet Earl Grey, the British minister for colonial possessions, but was refused. When a constitution was promulgated the following year, it incorporated some of Fox's ideas, but was not satisfactory to him.
Fox appears to have changed his views somewhat regarding Māori land rights, as he strongly opposed the government's policy on that issue. He blamed Stafford's administration, along with Governor Thomas Gore Browne, for the wars in Taranaki, which broke out when a Māori chief refused to sell his land. Fox was widely believed to have converted to support of the Māori, although many modern historians claim that his opposition to land seizure was due to a pragmatic wish to avoid war, not a change of philosophy. Lack of evidence makes it difficult to tell which was the case.
Fox, after becoming increasingly involved in a dispute with Grey over responsibility for policy towards Māori, lost a vote of confidence in 1862. The following year, he returned to government, but only as a minister - the premiership went to Frederick Whitaker. Fox appears to have had little to do with the policies of this government, which involved considerable confiscations of land from the Māori. After his term as a minister ended, Fox and his wife travelled in Australia for several years.
Upon returning to New Zealand, Fox was encouraged by the Opposition to return to politics, which was once again dominated by Fox's rival Edward Stafford. Fox was elected to parliament, and relaunched his attack on Stafford's policies on Māori relations and provincial affairs. Fox defeated Stafford in 1869, taking the premiership for the third time. Fox set about reducing military activities, and ceased any major attempts to engage the Māori with force. Increasingly, however, Fox found himself overshadowed by his treasurer, Julius Vogel. Vogel's extensive plans for the development of New Zealand, involving borrowing money to finance public works, soon became the most prominent feature of Fox's government, but had little to do with Fox himself. Eventually, Fox began to abandon his leadership role within the government, and the resulting disunity allowed Stafford to defeat Fox in 1872.
After this, Fox decided that he would not seek further office. His role in politics, however, was not quite over - when George Waterhouse, Stafford's successor, suddenly resigned, Fox was called upon to assume the premiership as a "caretaker" until a new leader was found. When Julius Vogel returned to New Zealand from an overseas trip, Fox stepped down, and Vogel's premiership began.
He was the Member of Parliament for Wanganui in the 2nd parliament (1855-60), then Rangitikei in the 3rd, 4th & 5th parliaments (1861 to 1865 when he resigned; 1868-70; 1871 to 1875 when he resigned), then Wanganui again in the 6th parliament (1876 to 1879 when he was defeated), then Rangitikei again (1880 to 1881 when he was defeated).
Fox subsequently became involved in movements against alcohol. He was again briefly the Member of Parliament for Rangitikei (1880 - 1881), but this was no longer the primary activity of his life. Fox also continued to undertake considerable physical exercise, climbing Mount Taranaki in 1892 (aged eighty). He died on 23 June 1893.