Moncreiffe was the son of Lieutenant-Commander Gerald Moncreiffe, RN, and Hilda, daughter of the Comte de Miremont. He succeeded his cousin as 11th Baronet and Chief of Clan Moncreiffe in 1957. Educated at Stowe School, Heidelberg, and Christ Church, Oxford, as a cadet officer Moncreiffe trained with Derek Bond and Patrick Leigh Fermor, he later served in Scots Guards during the Second World War, then as attaché at the British embassy in Moscow, before studying Scots Law at the University of Edinburgh. He was awarded a PhD with a thesis on the Origins and Background of the Law of Succession to Arms and Dignities in Scotland, which was published as a monograph in 2010. A prominent member of the Lyon Court, Moncreiffe held the offices of Falkland Pursuivant, Kintyre Pursuivant, Unicorn Pursuivant, and Albany Herald. He wrote a popular work about the Scottish clans, The Highland Clans, and with Don PottingerSimple Heraldry, Cheerfully Illustrated, Simple Custom, and Blood Royal, but his interests also extended to Georgian and Byzantine noble genealogies. Lord of the Dance, A Moncreiffe Miscellany, edited by Hugh Montgomery-Massingberd encompassed his genealogical world-view. He was elected a Fellow of the American Society of Genealogists in 1969. He was an incorrigible snob; he even called himself Master Snob. He took silk because very few barristers specialised in heraldic matters and he wished to highlight the importance of this field of speciality. He was a frequent writer of amusing and often illuminating letters to newspapers, particularly The Daily Telegraph, and provided the introduction to Douglas Sutherland's satirical book The English Gentleman. He held membership in many London clubs and founded his own club in Edinburgh, called Puffin's Club, – the name was taken from the nickname of Sir Iain's first wife, 'Puffin', Diana Hay, 23rd Countess of Erroll. It was and continues to be a weekly luncheon club, which between the early 1960s and late 1990s met in Martin's restaurant in Rose Street Lane North, Edinburgh. It still meets monthly in London and Edinburgh. The membership was and is as varied and eccentric as its founder. Ex-King Zog I, King of Albania paid his founding subscription of £5 in 1961, but died before he could attend. The actor Terence Stamp, Sir Nicholas Fairbairn, Sir Fitzroy Maclean and Lord Dacre all attended with varying frequency. It was said that at some point half the crowned heads of Europe were on the list.
Family
Moncreiffe married twice. Firstly Diana Hay, 23rd Countess of Erroll, whom he married on 19 December 1946 at St Margaret's, Westminster. He and Lady Erroll were one of the few couples who both held titles in their own right. They had three children, see Diana Hay, 23rd Countess of Erroll
Hon. Peregrine David Euan Malcolm Moncreiffe of that Ilk, Chief of Clan Moncreiffe, married in 1988 Miranda Mary Fox-Pitt.
Lady Alexandra Victoria Caroline Anne Moncreiffe Hay, married Jolyon Connell ; they have two daughters, Flora Diana Catharine Cecelia Connell and Ciara Edith Elisabeth Connell. Lady Alexandra also has a son, Ivar Francis Grey de Miremont Wigan.
Moncreiffe's first marriage was dissolved in 1964 and in 1966 he took as his second wife Hermione Patricia Faulkner, daughter of Lieutenant-Colonel Walter Douglas Faulkner by his marriage to Patricia Katherine Montagu Douglas Scott, the present Dowager Countess of Dundee.
