Born 5 March 1933 in Penrith, Australia, the second of four children of William Henry Maitland Holmes, a farmer from Berowra, and Ethel Vea Jay from Bellingen, Keith Holmes started his schooling at Berowra Primary School where he fell under the influence of a Mr. Dawson who instilled in him a lasting interest in natural history. At Homebush Boys High School he was inspired by the principal, Dr. Watson, who had been to Antarctica as a geologist. On leaving high school he joined the CSIRO for a year and attended night school courses in chemistry at the University of Technology in Sydney. From here he joined the family dairy farm at Berowra and moved with it to Raleigh, New South Wales in 1952. In 1958 he went to the United States and visited Maryland and Minnesota under the Young Farmers Exchange program.
From the 1960s Holmes had been collecting fossils from every site in Australia that he had been able to access, and particularly Glossopteris specimens from the Dunedoo formation in the western part of the Sydney Basin. When his farming operations moved to Wellington in New South Wales to start a beef herd of Aberdeen Angus cattle, he finally managed to put together his first paper for publication. The South African palaeobotanist, Dr. Edna Plumstead of the Bernard Price Institute for Palaeontological Research, who had focused world attention on the glossopterids of the Permian and Triassic as supporting evidence for continental drift, acted as referee for this first paper - Holmes had met Dr. Plumstead at the Conference on Stratigraphy and Palaeontology in Canberra in I973.
In 1980 Holmes became an Honorary Research Fellow in the Geology Department of the University of New England in Armidale and worked on the taxonomy of the Middle Triassic Nymboida Flora. His collection of approximately 3 000 specimens, supplemented by those of Gould, Flint and Retallack, is one of the most comprehensive Gondwanan Triassic floras, and is preserved in the Australian Museum. Parts 1-7 of the Nymboida Flora have been published, as well as papers dealing with early eucalypts from the Tertiary, cycads and conifers. Holmes enjoyed close relationships with the Australian Museum and his mentor, the former director, Dr. A.B. Walkom, as well as Dr. Rod Gould of the University of New England.
After his wife's death in 1998, Holmes left the management of his farming operations to his daughter 'Netta' and started visiting palaeobotany centres all over the world. In 2005 he was elected Fellow of the Linnean Society of New South Wales. He is an active member of Rotary International and staunch conservationist, having been delegate to the Nature Conservation Council of New South Wales He was chairman of the Burrendong Arboretum Trust from 1985 to 1995, as well as chairman of the Mount Arthur Reserve Trust near Wellington.