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Life and career 
Lees-Milne came from a prosperous manufacturing family and grew up in Wickhamford, Worcestershire. He attended Lockers Park School in Hertfordshire, Eton, and Oxford University. From 1931 to 1935, he was Private Secretary to George Lloyd, 1st Baron Lloyd of Dolobran.
In 1936 he was appointed secretary of the Country House Committee of the National Trust. He held that position until 1950, apart from a period of military service from 1939-1941. During that time he was a regular contributor to the Trust's member newsletter, penning various features. He was instrumental in the first large-scale transfer of country houses from private ownership to the Trust. After resigning his full-time position in 1950, he continued his connection with the National Trust as a part-time architectural consultant and member of committees.
Lees-Milne was visiting Diana Mosley when King Edward VIII abdicated. His visit there was to examine the seventeenth-century house she and her husband Sir Oswald Mosley were then renting; he recorded later how he and Diana (her husband was in London) had listened to the King's broadcast abdication speech with tears running down their faces. He had loved her brother Tom Mitford at Eton, and was devastated when Tom was killed in action in Burma in 1945.
He married Alvilde Chaplin, neé Bridges, a prominent gardening and landscape expert, in 1951. Alvilde Lees-Milne died in 1994. Both Lees-Milne and Alvilde were bisexual, and for a period Alvilde had lesbian affairs with Vita Sackville-West and Winnaretta Singer, among others.
After thirteen years living at Alderley Grange, Wotton-under-Edge, Gloucestershire and a brief sojourn in Bath, he and Alvilde resided after 1974 at Essex House on the Badminton estate, also in Gloucestershire, while he worked most days in William Thomas Beckford's library at Lansdown Crescent. He was a Founding Trustee of the Beckford's Tower Trust, established in 1977 to preserve and maintain the building and its collection for public benefit.
He was a friend of many of the most prominent British intellectual and social figures of his day, including Nancy Mitford, Harold Nicolson, about whom he wrote a two-volume biography, Deborah Mitford, and Cyril Connolly.
From 1947 Lees-Milne published a series of architectural works aimed primarily at the general reader. He was also a diarist, and his diaries were published in many volumes and were well received, in later years attracting a cult following. His other works included several biographies and an autobiographical novel.
In 1993 Lees-Milne was offered a CBE in the New Year's Honours, but declined it.
Lees-Milne died in hospital at Tetbury on 28 December 1997. His ashes, together with those of his wife, Alvilde, were scattered in the grounds of Essex House.
Selected bibliography 
- The Age of Adam (1947)
- The Tudor Renaissance (1951)
- The Age of Inigo Jones (1953)
- Roman Mornings (1956)
- Earls of Creation: Five Great Patrons of Eighteenth-Century Art (1962)
- St Peter's: The Story of Saint Peter's Basilica in Rome (1967)
- English Country Houses: Baroque, 1685-1715 (1970)
- Another Self (1970), an autobiographical novel
- Prophesying Peace (1977)
- Round the Clock (1978)
- Harold Nicolson: A Biography, 2 vols. (1980-1)
- Caves of Ice (1983)
- The Last Stuarts: British Royalty in Exile (1984)
- Midway on the Waves (1985)
- The Enigmatic Edwardian: The Life of Reginald, 2nd Viscount Esher (1986)
- William Beckford (1986)
- The Bachelor Duke: A Life of William Spencer Cavendish, 6th Duke of Devonshire, 1790-1858 (1991)
- Ruthenshaw (1994), fiction, a ghost story.
- Fourteen Friends (1996)
- Ancestral Voices 1942-3 (1975)
- Deep Romantic Chasm: Diaries, 1979-81 (2000)
- Ceaseless Turmoil (2004)
- The Milk of Paradise (2005)
- James Lees-Milne, Ancestral Voices (London: Chatto & Windus, 1975), 6n1
- Review of Diaries, 1971-1983 by James Lees-Milne, Sunday Express, retrieved 18 November 2007
- Michael Bloch 'James Lees-Milne - The Life' - see Sources
- 24 January 2012
- Michael Bloch, James Lees-Milne: The Life (John Murray, 2009), ISBN 978-0-7195-6034-7). An authorized biography.