Thistle Yolette Harris was born in Sydney, Australia, a daughter of Charles Thomas Harris, later the general manager of the Sydney Morning Herald, and his wife Ilma Richardson. She was educated at North Sydney Girls’ High and Redlands School, where one of her influential teachers was Constance Le Plastrier. Ms. Le Plastrier fostered Thistle's interest in native plants and took her to a meeting of the Naturalists’ Society of New South Wales, where she met David Stead, a pioneering conservationist and marine biologist, and began a lifetime of commitment to the natural environment. She studied science at the University of Sydney and graduated in 1924. After several years as a science teacher in secondary schools, she became a lecturer in science education at Sydney Teachers' College in 1938. She was involved in many conservationist organizations and compiled and edited the publication Junior Tree Warden for many years. Her first book was Wild Flowers of Australia (1938), hailed as a watershed publication by some critics. In 1939, she began living with the married David Stead; they married after the death of his first wife in 1951. During the 1940s, she earned a master's degree in education at the University of Melbourne and studied for a diploma in Landscape Design at the University of New South Wales. She served as President and Honorary Secretary of the Wildlife Preservation Society of Australia, and worked on the Society's journal Australian Wild Life, issued intermittently from 1934. She wrote many more books on Australian flora and their cultivation in suburban gardens. In 1963, she established the Wirrimbirra Sanctuary at Bargo, south of Sydney, which became a center for education and research, as a memorial to Stead, who died in 1957.
She was named a Member of the Order of Australia in 1980.