According to family information John Williams Harris was born in Cornwall, England, in 1808. He joined the Royal Navy while still a boy, although ill health forced him to quit. He emigrated to Australia to join relatives in 1820 or 1821, and spent about 10 years there, working in a counting-house and on a sheep station. But it was his work with the merchant J. B. Montefiore's company which led to his arrival in New Zealand. Harris, George White and Tom Ralph were sent out on the company's ship Darling in February 1831 to establish a flax-trading business; they arrived at Poverty Bay in May of that year.
Harris established himself first at Awapuni Lagoon, in the lower reaches of the Waipaoa River, before moving to the west bank of the Turanganui River later in 1831; he eventually purchased the trading-post from Montefiore. Like many other Pakeha traders, Harris owed much of his success to his connections with the Maori; he came under the protection of Paratene Turangi, and probably in 1832 or 1833 married Tukura-a-Rangi, a relative of Ngati Porou leader Te Kani-a-Takirau. Tukura and Harris had two children, Edward Francis and Henry.
In June 1831 Harris made the earliest European land purchase in Poverty Bay, a section of a little more than an acre on the banks of the Turanganui River. He purchased another block named Opou, near Awapuni, for which two deeds were signed in 1839 and 1845; there he built his house. He leased nearby sections, and may also have owned land at Ruataniwha in Hawke's Bay. Harris was active in early pastoral farming and for 20 years had the biggest holding on the flats, but claims that he was the first to introduce sheep, cattle and horses to the district have been disputed.
In 1837 Harris travelled to Sydney to purchase goods needed for the establishment of the first whaling station in Poverty Bay. (It was on this voyage that he took with him a bone from a large bird; Richard Owen was to use it in 1839 to verify the previous existence of the moa in New Zealand.) In association with Thomas Halbert he established the station next to his store on the west bank of the Turanganui River in 1837. He moved it to the Kaiti side of the river in 1838 and around the coast to Papawhariki in November 1838. In 1842 he retired from active participation in the whaling business to settle on his land at Opou.
The first Poverty Bay census in 1851 showed that Harris was the most substantial settler in the district. He owned five of the twenty weatherboard buildings in the district and considerably more livestock than his fellow Pakeha. His acquisition of land had been facilitated by his care in maintaining good relations with the Poverty Bay Maori. After Tukura's death, some time after 1851, Harris's association with her people continued to be mutually beneficial.
As a key person in the settlement, and a Pakeha who could move easily among the Maori, Harris became a major source of information about Poverty Bay for Donald McLean and was appointed an arbitrator in disputes between Maori and Pakeha, an indication of his standing with both races. During the 1860s he kept McLean informed on the movements of Maori belligerents in the district. His house at Opou was burned down during Te Kooti's raid of 1868. Harris's letters to McLean have been preserved and show him to have been a well-educated and literate man, unlike many of his fellow traders.
Harris maintained a good relationship with local missionaries William and Jane Williams, who held some services in his house. At one time, however, they broke off the relationship because William Williams could not 'countenance his mode of living and shameless practices'. Harris, wrote Jane Williams, was living in a 'licentious manner.'
For the first 30 years of Poverty Bay's settlement Harris was its most prominent Pakeha citizen. One of his most important contributions was in bringing other traders to the district, either as his employees or as businessmen in their own right attracted to a growing area. During the 1860s competition from George Read forced Harris's own trading activities into the background.
On 30 December 1854 Harris married Jacintha Adelaide Hargraves at Auckland. She later moved back to Auckland with their two children, Harold and Bertha. Harris seems to have become increasingly depressed as a result of this separation. He committed suicide while on a visit to Auckland, on 4 February 1872.