According to family information James Reddy Clendon was born in Deal, Kent, England, on 1 October 1800; he was baptised there on 22 October. He was the son of George Clendon, pilot of the Cinque Ports, and his wife, Elizabeth Chitty. James Clendon began his business career as a ship owner in London, in partnership with his brother John Chitty Clendon. He married Sarah Hill at Port Jackson (Sydney) on 2 October 1826. Their first child, James Stephenson Clendon, was born in London in January 1827.
In 1828, as captain of the City of Edinburgh, he transported female prisoners to the convict settlement at Port Jackson, and having discharged these, he spent some time on the Australian coast before proceeding to New Zealand for spars. Sarah Clendon was with him on this voyage, and in January 1829, at sea off Hokianga, she gave birth to their second child, Eliza. They returned to London in 1830.
Having purchased the schooner Fortitude, Clendon and his partner, Samuel Stephenson, sailed for New Zealand, arriving at the Bay of Islands on 31 August 1832. While there in 1830 Clendon had arranged with Pōmare II and Kiwikiwi, of Ngāti Manu, to acquire some 220 acres at Okiato, about four miles upstream from the growing village of Kororāreka (Russell). Here he and his family settled, and set up a trading station to supply the needs of the rapidly growing Pacific whaling fleet. In 1837 he added another 80 acres to his Okiato estate, and a year later purchased 3,342 acres at Manawaora, on the seaward coast. This became the Clendon farm.
With the Fortitude Clendon and Stephenson traded for provisions, flax and timber, and in 1833 entered Hokianga Harbour to load timber at George Nimmo's Motukauri establishment. Misfortune overtook the vessel here when it stranded and was plundered by local Māori. This caused a minor war between rival tribal groups, but the Fortitude survived and was floated off safely.
Clendon then became involved in Bay of Islands politics. In 1834 he was one of the signatories to a letter to James Busby, the British Resident, demanding punishment of a Māori who had fired on Busby when surprised by him while robbing an out-building. In 1835 Clendon publicly drained all his rum casks, apparently hoping to encourage sober habits among the riotous lower orders at the Bay. It was an unsuccessful sacrifice, however, since the unrepentant were making much money out of the trade. In this period, too, he supported Busby in his efforts to unite the northern tribes to thwart the pretensions of Baron Charles de Thierry, who hoped to establish himself as 'sovereign chief' at Hokianga. Clendon also signed the 1837 petition to William IV, praying for protection against increasing local lawlessness. An indication of his standing in the community was his appointment as president of the New Zealand Banking Company, which opened New Zealand's first bank at Kororāreka in September 1840.
Clendon's commercial pre-eminence was assured when, on 12 October 1838, he was empowered to act as United States consul in New Zealand; but he could not assume full consular status, there being no government to which he could be accredited. However, he did gain virtually all the business of visiting American ships, and since he recorded the arrival of 151 American ships between 1839 and 1841, it was undoubtedly a profitable appointment.
This favourable progress suffered after the arrival of Captain William Hobson in January 1840. Clendon supported him in his negotiations for the cession of sovereignty to the British Crown, but the signing of the Treaty of Waitangi dealt a cruel blow to his American business, since it adversely affected United States interests, politically and financially. But Clendon did sell his Okiato property to the newly fledged administration, for the government establishment. Hobson, now lieutenant governor, took possession and had the land surveyed as a town. He named this Russell, and it remained the seat of government until February 1841. Owing to administrative problems, Clendon received only a small part of the purchase price and was obliged to accept in settlement a 10,000 acre block south of Auckland.
Having delegated his consular functions after the signing of the treaty, Clendon relinquished them entirely in 1841. He then resorted to farming his Manawaora land and entered his phase as a public servant. In 1840 he had been appointed a justice of the peace and had become a member of New Zealand's first Legislative Council. As the administration stabilised, Clendon assumed the duties of police magistrate at the Bay of Islands in 1845, together with other minor government functions, and during the war with Hōne Heke advised the British military on local matters. He was present in this capacity at the attack on Ōhaeawai. After the war he continued in office at Kororāreka, by then renamed Russell, and conducted a census of the European population of the north in 1846. From about that time his magisterial jurisdiction extended to include Hokianga.
In 1855 Sarah Clendon died at the Bay of Islands, leaving her husband with six children. On 9 January 1856 at Paihia, Clendon married Jane Cochrane, the daughter of Dennis Cochrane of Hokianga and his late wife, Takatowai Te Whata. James and Jane had eight children. Early in 1862, after a brief residence at Kerikeri, the Clendons settled at Rāwene. As magistrate there under the Native Circuit Courts Act, with his son James acting as clerk of court, Clendon dispensed justice in informal surroundings and in conformity with Māori conceptions. His reputation in this respect was already well established; while resident magistrate at Russell in 1858, he had visited Hokianga and successfully settled 67 European–Māori disputes.
However, the Resident Magistrates' Act 1867 repealed the Native Circuit Courts Act, and Clendon retired. He now reverted to his former occupation as a merchant, taking up licences to sell beer, wine and spirits at Rāwene; these licences were current from 1869 to 1872. On 24 October 1872 he died at Rāwene.
Clendon's house still stands at Rāwene. Built in the late 1860s, it is now owned by the New Zealand Historic Places Trust and is open to the public. Jane Clendon, only 34 years old at James's death, lived until 1919.