Ernest Valentine Sanderson was born in Dunedin on 8 February 1866, the second of five children of Jane Sanderson and William Walter James Spreat, a lithographic draughtsman. By 1874 the family had moved to Wellington, where Val, as he was known, attended Wellington College and then began work as a clerk with the Australian Mutual Provident Society. William Spreat prospered, but his intemperate behaviour eventually drove the family to leave him. At his entreaty they returned to his house while he lived separately in a stable, but William's health and temper worsened until in July 1893 a violent attack on his sons Val and Louis ended tragically with the latter shooting him. As he lay dying he signed a statement exonerating his sons; the coroner's jury subsequently agreed that it was a 'justifiable homicide'.
In 1900 Val Sanderson enlisted for service in the South African War, and demonstrated his organisational ability as quartermaster for his regiment. He then briefly returned to his job at the AMP until the final division of his father's estate gave him the capital for a series of business ventures. The most successful of these was in partnership with Godfrey Magnus, initially to import bicycles; as Magnus, Sanderson and Company they soon expanded into the new motor trade. On 6 January 1904 Sanderson married Emily Louisa Cooper at Lower Hutt; they were to have no children.
In 1913 Sanderson travelled to Britain and Europe pursuing various business opportunities but he was back in New Zealand when war was declared the following year. He immediately tried to enlist again, understating his age by five years to bring it under 45. After several applications he was accepted and sent to Egypt as a quartermaster but served only seven months before he was repatriated and discharged ill. Back in New Zealand, Captain Sanderson (as he was henceforth known) returned to health but took some time to regain direction. He had already become estranged from his wife and eventually, in 1921, they divorced. About this time he withdrew from Magnus, Sanderson and Company and found a new cause to devote himself to.
Before the war Sanderson had been involved in a public controversy over the state of the flora and fauna reserve on Kapiti Island, which had become a focus for a growing sentiment of national pride concerning native bush and birds. In 1921 he revisited Kapiti and found that still nothing had been done to fence off the reserve area or remove the sheep and goats which were eating away the bush. Sanderson had a keen sense of duty and expected authorities to live up to their responsibilities. He lambasted the ministers and departments responsible for Kapiti and demanded that more be done to protect it.
In 1923, at the suggestion of Sir Thomas Mackenzie, an older crusader for such issues, Sanderson expanded his campaign beyond Kapiti. There was already a group, the New Zealand Forestry League, promoting protection of native forests nationally, so he launched a complementary New Zealand Native Bird Protection Society. Mackenzie was elected president but Sanderson remained the driving force. He built up a membership of supporters and adroitly organised publicity to bring native birds to popular attention. As the forestry league faltered he extended his society's scope and eventually in 1935 it was renamed the Forest and Bird Protection Society of New Zealand.
Sanderson's campaigns through the society extended beyond forest and bird protection. As a businessman he pressed for more efficient administration of wildlife protection. From his reading of American literature he warned of the dangers of soil erosion and promoted the value of native bush as a protection against it. From the botanist Leonard Cockayne he took up the idea that the New Zealand bush was not adapted to withstand browsing by deer and other introduced animals, and thus campaigned vigorously against the 'deer menace'.
By the 1930s the Native Bird Protection Society was well established. After a succession of largely figurehead presidents, in 1933 Sanderson himself took this position. At nearly 70 he was a craggy, rather stern figure. He had established a home at Paekakariki, opposite Kapiti Island, and on 10 October 1934, at Napier, he married Nellie Milne, more than 30 years his junior; they had two daughters. He still spent three days a week on society business and remained in command up to his death on 29 December 1945. He was survived by Nellie and their children. The Forest and Bird Protection Society he had built up continued to flourish as New Zealand's premier conservation organisation.