Fox was born in Westoe Village, South Shields, the son of George Townshend Fox, deputy lord lieutenant of Durham. After attending Oxford and being called to the bar, he emigrated to New Zealand at the age of 30. There he bought an estate in Wanganui, which soon became known as Westoe. In 1843, the New Zealand Company appointed him residential agent and he headed a delegation to London to demand a constitution. The Colonial Office refused to see him.
The first New Zealand parliament eventually met in 1854, and the second in 1856 when Fox became prime minister for 13 days. He managed to serve over a year in 1861-62 and a similar period in 1863-64. In January 1869, Fox formed a ministry which lasted until September 1872. Thereafter he took little part in politics. His generous nature and quick impulsive temperament made him better suited as a fighting opposition leader that when on the defensive as a minister. Fox's achievements lie in securing self-government for New Zealand and arranging a lasting peace with the native tribes. His book The War in New Zealand (1860) vindicates the colonists from the charge of provoking war with the Maoris in order to seize their lands.