From the 1860s Hunter was interested in conservation of public open spaces, and worked with other pioneers in this field, including Octavia Hill and Hardwicke Rawnsley. After acting as adviser to Hill in her campaigns to save Hampstead Heath and other open spaces, he worked with Rawnsley to save land in the English Lake District from industrial development. In 1893 the three campaigners agreed to set up a national body to acquire vulnerable properties and preserve them for the nation. At Hunter's suggestion it was entitled the National Trust for Places of Historic Interest or Natural Beauty, generally known simply as the National Trust. Hunter was the founding chairman of the trust's executive board.