Wesley Spragg was born on 13 February 1848 at Madeley, Shropshire, England, the son of Mary Hill and her husband, Charles Spragg, a blacksmith. He was educated at the Wesleyan school in Madeley, and emigrated to New Zealand with his family on the Ulcoats, arriving at Auckland on 22 January 1864.
Of the first 20 years of Spragg's life in New Zealand little is known. He is said to have spent some time on the Thames goldfields. His interest in the dairy industry began in Auckland in 1883 or 1884, when he was in charge of the butter department of the New Zealand Frozen Meat and Storage Company, responsible for purchasing and reselling surplus farm-produced butter. The quality was variable, and Spragg soon realised that improvement required a move to factory-made butter. He organised the farmer suppliers, and manufacturing began at the butter factory on the premises of the Auckland Frozen Meat and Storage Company. However, the quality of milk supplied proved unreliable, profits remained low, and manufacturing ceased when the frozen meat company was wound up and its property sold.
Still convinced of the good prospects for factory-made butter, Spragg obtained the support of John Bycroft, the flour and biscuit manufacturer, and in 1886 the New Zealand Dairy Association was formed with Spragg as general manager. Its butter factory was located at Pukekohe from 1889. Good dairying seasons in Australia closed off that market, but Spragg successfully turned to new markets in England. When the depressed state of the economy in the early 1890s once more threatened the new industry, he purchased the company as joint proprietor with the London agents, Lovell and Christmas. In 1896 the New Zealand Dairy Association purchased their Waikato competitors, Reynolds and Company, taking over the famous Anchor brand. By the turn of the century 1,200 tons of butter were being produced annually, with 800 tons sent to London and 400 tons to the local market.
Reflecting the increasing popularity among farmers of co-operative ownership, in 1901, with Spragg's support, 847 shareholders bought the New Zealand Dairy Association for £40,000. Spragg continued as managing director until 1912, when he resigned following a serious accident and became chairman of directors until his retirement in 1915.
From 1909 the New Zealand Dairy Association faced increasing competition from the Waikato Co-operative Dairy Company, led by William Goodfellow, which encouraged home-separation as opposed to the collection of milk from farms for processing at creameries. However, in 1910 the association purchased the Waikato portion of Ambury, English and Company's butter manufacturing, and with it took over their suppliers who provided home-separated cream. Spragg remained opposed to home-separation on grounds of quality and profitability, but by 1915 the New Zealand Dairy Association claimed to produce more butter from home-separation than any other company in New Zealand: some 1,400 tons annually, over a quarter of the company's total. In 1919, four years after Spragg's retirement, the New Zealand Dairy Association merged with Goodfellow's Waikato dairy and cheese companies, and subsequently with the Thames Valley Co-operative Dairying Company, to form the New Zealand Co-operative Dairy Company.
Wesley Spragg's business acumen and reputation for efficiency laid the foundation for the dairy industry in Auckland province, and during a period when transport was difficult, technology limited and supply unreliable in quantity and quality, he retained the confidence of farmers because of his sound business judgement and integrity. In 1888 Spragg was the first to use parchment paper for wrapping butter, and in 1913, in order to maintain a competitive market for whole milk, was responsible for the introduction of casein manufacture in Auckland.
On 29 January 1878, at Auckland, Wesley Spragg had married Henrietta Neal, the daughter of an Auckland tea merchant. They had five daughters, and a son who died in infancy. Henrietta died in 1889 and on 30 July 1891 at Auckland, Spragg married her sister, Annie Dearnley Neal; they had two sons, one dying in infancy. Wesley Spragg had a strong love of native bush and donated Kaitarakihi Park in the Waitakere Range to the people of Auckland in memory of his third son, Wesley Neal, who was killed in action in Egypt in 1918. He donated two more areas of native bush between 1919 and 1928, making a total of 812 acres.
A lifelong concern for the welfare of others was shown by Spragg's reputation as a fair and generous employer. He was a prominent layman in the Congregational church and a strong supporter of the temperance movement. From 1908 to 1915 he was president of the New Zealand Alliance, and for 20 years president of the Auckland Province No-License Council. Both Henrietta and Annie Spragg shared in these social reform interests. Henrietta Spragg was also prominent in the women's suffrage movement, organising petitions in 1886 and 1887. Annie Spragg died in 1934, four years after her husband, who died at his home in Auckland on 15 August 1930.