His studies were interrupted by the Second World War, in which he served as an Adjutant (Captain) in the Northumberland Fusiliers, initially training recruits to shoot, but later joining his regiment in Italy, where he contracted typhoid.
In 1946, while still in uniform, he married Winifred Brown, whom he had met at King's College before the war.
[1] After graduation, Green was appointed Assistant Lecturer at Birmingham University before joining the Royal Botanic Garden, Edinburgh in 1952.
In 1961, he moved to the Arnold Arboretum of Harvard University to become Horticultural Taxonomist, but returned to England in 1966 to join the Herbarium of the Royal Botanic Gardens Kew, ultimately rising to the rank of Deputy Director and Curator.
He also travelled widely, notably to the islands of the western Pacific, whose flora had preoccupied him during his career at Kew.