Sir Rupert Iain Kay Moncreiffe of that Ilk, 11th Baronet, CVO, QC (9 April 1919 – 27 February 1985) was a British officer of arms and genealogist. He used various forms of his name: His columns for Books and Bookmen were signed Iain Moncreiffe; Royal Highness is by Sir Iain Moncreiffe of that Ilk, Bt.; Simple Heraldry is by Sir Iain Moncrieffe of Easter Moncreiffe. Like other Scottish landowners, and other baronets, he distinguished himself from other Moncreiffes by referring to his estate: of that Ilk is Scots for "of the same ", since his estate was Moncreiffe Island itself. (Easter Moncreiffe was the name of his house; Moncreiffe House burnt down in 1957, and its ruins were inherited by his cousin's daughter.)
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 he trained with Derek Bond (actor) and Patrick Leigh Fermor, he later served in the 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, where he graduated Ph.D. with a thesis on the Scots law of succession to peerages.
A prominent member of the Lyon Court, he held the offices of Falkland Pursuivant (1952), Kintyre Pursuivant (1953), Unicorn Pursuivant (1955), and (from 1961) Albany Herald. He wrote a popular work about the Scottish clans, The Highland Clans (1967), Simple Heraldry, Cheerfully Illustrated (1953), Simple Custom (1954), and Blood Royal (1956) with Don Pottinger, 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 an incorrigible snob; he even called himself Master Snob. He took silk (relatively late in his career), 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 (1978). He held membership in many London clubs and founded his own club in Edinburgh, called Puffin's, after his first wife, Diana Hay, 23rd Countess of Erroll, whom he married on 19 December 1946 at St Margaret's, Westminster. He and his wife were one of the few couples who both held titles in their own right. They had three children:
- Merlin Sereld Victor Gilbert, styled Lord Hay, later 24th Earl of Erroll (b. 20 April 1948).
- Hon. Peregrine David Euan Malcolm Moncreiffe of that Ilk, Chief of Clan Moncreiffe (b. 16 February 1951), married in 1988 Miranda Mary Fox-Pitt (b. 29 December 1968).
- Lady Alexandra Victoria Caroline Anne Moncreiffe Hay (b. Edinburgh, 30 July 1955), married Jolyon Connell (b. Edinburgh, 16 May 1952); they have two daughters, Flora Diana Catharine Cecelia Connell (b. London, 24 February 1990) and Ciara Edith Elisabeth Connell (b. London, 28 March 1994). 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.