Sassoon was the only child of the poet Siegfried Sassoon and Hester Sassoon (née Gatty), and was born in London, where his parents had rented a house in order to be closer to specialist help than at their home in Heytesbury, Wiltshire. He was christened at St Martin-in-the-Fields by Rev Dick Sheppard.
George's father wrote playfully to Max Beerbohm in November 1936: "Will he, I wonder, become Prime Minister, Poet Laureate, Archbishop of Canterbury, or merely Editor of The Times Literary Supplement? Or Master of The Quorn? Or merely Squire of Heytesbury?"
In 1947, Sassoon's parents separated, and he thereafter spent much of his childhood with his mother on the Scottish island of Mull. He was educated at Greenways Preparatory School, Ashton Gifford House, near Codford, then at Oundle School and King's College, Cambridge.
He was noted for his prodigious linguistic ability, learning languages including Serbo-Croat, Hebrew, Aramaic and Klingon. He investigated extraterrestrial phenomena and helped his mother to run a sheep farm on Mull. Between 1978 and 1980, he published three books, two of which were about his theories on extraterrestrial visitations, and also spoke at conferences on alien phenomena.
After his father died in 1967, Sassoon inherited and occupied his father's large country house, Heytesbury House at Heytesbury, Wiltshire. He found it much neglected and worked to restore it, and also battled unsuccessfully to stop a planned A36 bypass from going through the park of the house; in the process, he sold many of his father's papers to raise additional funds. The southern part of the park was subject to a compulsory purchase order in 1985 to allow construction of the bypass, separating the house from its cricket field and requiring a new entrance to be made from the west. He sold the house in 1996 after a serious fire and moved to a smaller property in the nearby village of Sutton Veny; but spent part of the year on Mull, where he had inherited his mother's property Ben Buie on her death in 1973.
Sassoon married four times: Stephanie Munro, at Inverness in 1955 (dissolved 1961); Marguerite Dicks in 1961 (dissolved 1974); Susan Christian-Howard in 1975 (dissolved 1982); and lastly, Alison Pulvertaft in 1994.
George's daughter and son by his third marriage, Isobel (born 1976) and Thomas (born 1978), were both killed in a road accident in Mendip, Somerset, in 1996.
Sassoon was something of a bon-viveur, well-known among other things for his playing of the piano-accordion. Among his other interests were cricket, the Antipodes, and amateur radio, where his call-sign was GM3JZK.