According to his own account William Walter Smith was born on 14 September 1852 at Hawick, Roxburghshire, Scotland. He was the son of Ellen Robson and her husband, Thomas Smith, a gamekeeper. From the age of 13 Smith worked his way up from apprentice to foreman gardener at a succession of English country houses. In 1875 or 1876 he emigrated to New Zealand and was employed by a colonial country gentleman, J. B. A. Acland, of Mt Peel station, Canterbury. He worked there until shortly before his marriage on 30 September 1880 to Mary Foreman, a dressmaker. The couple were married in Ashburton; they were to have at least seven children, three of whom died in childhood. Smith found further employment as a gardener in Canterbury and Otago but it seems to have been a rather precarious living.
Alongside his gardening work Smith made a habit of noting and collecting native plants and birds, ants and earthworms, Maori artefacts and moa bones. Initially he took the generally held view that with colonisation many native species would, like the moa, become extinct. Some of his observations and specimens were indeed the last of their kind and thus particularly valuable. When working at Albury estate near Timaru in the early 1880s, for instance, he discovered that the rare laughing owl ( Sceloglaux albifacies ) still lingered in limestone cliffs in the area. Scientists and collectors paid Smith up to three guineas – equivalent to several weeks' wages – for every one he could find.
Smith also began sending notes on natural history to both colonial and English journals. Despite his lack of scientific training or social standing he wrote with considerable assurance. When in 1890 the English entomologist Edward Meyrick refused to believe one of his reports Smith stood his ground; it was unscientific, he argued, to reject observations merely because they did not come from an accepted English authority. He did, however, leave the technical description of any new species he found to the recognised authorities, and was repaid in the customary fashion by having several species named after him.
In 1894 Smith gained a more secure position as resident custodian of the Ashburton Domain. He laid out gardens there and became a prominent member of the local beautifying and horticultural societies. He continued to write on natural history topics in general and also on the new idea, gaining ground at this time, that native birds and bush scenery were a national heritage worthy of preservation and protection. Accordingly, when the Scenery Preservation Act of 1903 was passed, Smith put his name forward and was appointed to the commission charged with putting the act into effect. This was the high point of his career. With the chairman, S. Percy Smith, and the other commissioners he travelled the country inspecting scenic and historic sites and making recommendations for their reservation. But the commissioners were perhaps too independent and too diligent; their recommendations and their expenses drew unfavourable comment in Parliament, and in 1906 they were replaced by a more compliant official board. William Smith was dropped from his position of importance as suddenly as he had been taken up.
From this time Smith and his wife followed separate lives. Mary prospered as the proprietor of a maternity home in Ashburton and later moved to live with a son in Masterton, where she died in 1922. William had moved north in 1906 and late the following year was appointed curator of reserves for the Palmerston North Borough Council. However, he would no longer play the humble servant and resigned in February 1908 after various disagreements with the mayor, Richard Essex, who later called him 'a common cabbage gardener'. In the following month, with Percy Smith's assistance, William obtained the position of curator of Pukekura Park, New Plymouth. There he developed the park, improving its fine vistas and planting native trees and ferns, while also pursuing his other interests. He successfully bred kiwi at the park. As a member of the Polynesian Society he loyally supported Percy Smith, its president, and served as secretary from 1911 until Percy Smith's death in 1922. He wrote and gave advice on a wide range of horticultural and natural history subjects; his advice was sought so often that in October 1920 the park board moved to limit the calls on his time. Again he felt demeaned and abruptly resigned.
Smith, now 68, was unable to obtain another position. By 1930 he was forced to apply for the old-age pension. This just covered his lodging-house board, but with assistance from his sons he maintained his independence. He could no longer afford to maintain his membership of scientific societies, but in 1931 the New Zealand Native Bird Protection Society, which he had strongly supported from its inception in 1923, tactfully made him an honorary life member. He remained active, a spare, bearded figure walking almost daily to read at the public library. He continued, until shortly before his death, to correspond with a wide circle of scientists, conservationists and the children who responded to his natural history articles. He died at New Plymouth on 3 March 1942.