He was born in Cambridge and studied in the University Botanic Garden at Cambridge University. For thirty years he was Gardens Adviser to the National Trust and in 1975 received the Order of the British Empire (OBE) for his work with that organization. He supervised the restoration of some of the most famous gardens in Great Britain including Mottisfont Abbey in 1957, Sezincote in 1968, and Eyford Park in 1976.
In addition to the OBE, Graham Thomas was one of the sixty-three holders of the Victoria Medal of Honour. He was awarded the Veitch Memorial Medal by the Royal Horticultural Society as well as their Gold Medal for his paintings and drawings, and the Dean Hole Medal by the National Rose Society. The Garden Writers' Guild gave him a Lifetime Achievement Award in 1996. In 1983, a yellow rose was named in his honor by rose breeder David Austin.
He was probably best known for his work in gathering and popularizing old and new shrub roses. In addition to having an encyclopedic knowledge of horticulture, Thomas was an artist whose paintings and drawings illustrate his many books. He was also a talented photographer, poet and musician who has been described as "one of the towering figures of the art and craft of gardening of all time".