Fenton was born in London, England, where he was baptised at St.Mary's church in Islington on 6 August 1824. He was educated at the Collegiate boarding school in Sheffield, in Yorkshire, and was articled to his uncle James Crossland Fenton who was a solicitor in Huddersfield. He went to Auckland, New Zealand, in 1850, and cultivated land on the Waikato River. In 1851 he was appointed by Sir George Grey (then Governor) to a position in the Deeds Office at Auckland, and subsequently became Resident Magistrate and Collector of Customs at Kaipara. In March 1856 he was made Native Secretary by Governor Browne; but his policy clashing with that of (afterwards Sir) Donald McLean, Chief Native Land Commissioner, he was compelled to resign, and was appointed Resident Magistrate at Whaingaroa (now Raglan). On 9 May 1857, as the result of a paper of suggestions which he had addressed to the Governor, he was summoned to act as magistrate at Waikato at a critical time in the difficulties preceding the Waikato war. His appointment, "at the earnest request of the natives," was the promise of a new departure on the part of the Government, who had resolved that the Maori should be governed by laws "enacted with their own consent," and instructed Fenton to prepare a code upon this understanding. Fenton proceeded to Waikato, but on 14 July 1857, Potatau accepted the kingship offered him by the malcontent tribes, and he was shortly afterwards withdrawn. From 1858 to 1864 Fenton acted as assistant law officer of the Crown. In 1861 he prepared the Domain Act, and in 1863 was charged with the working of the New Zealand Settlement Act.