Greta Barbara Stevenson was born on 10 June 1911 in Auckland, the eldest of four children of William Stevenson, a clerk, and his wife, Grace Mary Scott. In 1914 the family moved from Auckland to Dunedin, where Greta's father became managing director of the family's food-processing company, Irvine and Stevenson's St George.
Greta Stevenson was dux in 1924 at Maori Hill School and in 1927 at Columba College, which she attended from 1925 to 1928. She went on to the University of Otago in 1929 and was an outstanding botany scholar, graduating BSc in 1932 and MSc with first-class honours in 1933. Her awards included a Senior Scholarship in botany, the Sir George Grey Scholarship and the Lubecki Scholarship in applied science. A paper based on her MSc thesis, about a rare parasitic plant, Korthalsella, was published in part in the Transactions and Proceedings of the Royal Society of New Zealand in 1934.
During various botany and geology field trips Greta Stevenson became interested in tramping. She began alpine climbing in 1933 when, with Lella Davidson, she ascended the east peak of Mt Earnslaw at the head of Lake Wakatipu. This was the first significant climb in New Zealand by an all-women party. The following season Stevenson spent several weeks in the Mt Cook region. Here, with Alastair Duthie and guide Kurt Suter, she climbed a number of high peaks and made the first crossing of the Clarke Saddle. Stevenson was a strong, athletic and dependable organiser and leader.
In 1934 she travelled to London, and while holding a Shirtcliffe fellowship at the Imperial College of Science and Technology she completed her PhD and the college's diploma in mycology and plant pathology. On 16 October 1936 in London she married Edgar Cone, a research student in chemical engineering. They were to have two children. Greta retained an interest in mountaineering and rock climbing. She was a member of the Ladies' Alpine Club in England from 1935 to 1938, when she returned to New Zealand with Edgar.
After a brief time in Dunedin, Greta Cone based herself with her family in Wellington. While her children were young, she worked as an analyst for Wellington City Council and a soil microbiologist for the DSIR soil bureau, and taught science at various secondary schools.
Greta Cone was an assertive, independent individual, whose love of plants and enthusiasm for the outdoors made her an inspiring teacher. She was actively involved in many New Zealand botanical societies, where her knowledge and teaching skills were an asset. As a committee member of the Wellington Botanical Society she helped to initiate its Bulletin in 1941 and was president from 1944 to 1946. She was also a member of the New Zealand Alpine Club from 1934 until at least 1949, becoming a section committee member for Wellington from 1941 to 1942.
Greta Stevenson wrote many articles and scholarly papers on mycology and published three popular books on ferns and fungi, all with her own illustrations. Her most significant original research publication, illustrated with her own watercolours, was a series of five papers on the agaricales fungi (fleshy toadstools) of New Zealand, published in the Kew Bulletin between 1962 and 1964.
Greta and Edgar Cone separated about 1951, and were divorced in 1966. Greta retained her married name but published under the name Stevenson. After this time she moved restlessly from place to place. From 1954 to 1957 she was a Nuffield research fellow at the Cawthron Institute in Nelson, working on the symbiotic associations of New Zealand native plants. She returned to England in 1958, where she was a research assistant at the Imperial College of Science and Technology, London, then taught at various tertiary institutes there and in Sussex and Winchester. In 1970 she went back to Wellington and for 10 years worked as an unpaid research officer in botany at Victoria University. She was an active member of the New Zealand Women Writers' Society, serving on the executive and as a librarian from 1975 to 1977. From 1980 to 1981 Stevenson worked at the University of Canterbury's Botany Department. Here, while undertaking research, she conducted a number of workshops and study courses on larger fungi. She returned to England in 1986 and died in London on 18 December 1990, survived by a son and a daughter.