The son of Lieutenant-Commander Gerald Moncreiffe, RN, and Hilda, daughter of the Comte de Miremont, he succeeded his cousin as 11th Baronet in 1957. The baronetcy derived from the feudal barony of Moncreiffe, near Perth, Scotland. "Of that Ilk" means "of that same [place]", i.e. it is a contraction of "Moncreiffe of Moncreiffe" (a common mistake is to assume it means "with the same name" or "of the same type").
Educated at Stowe School, Heidelberg, and Christ Church, Oxford, he served in World War II in the Scots Guards, served as attaché at the British embassy in Moscow, and then studied Scots Law at the University of Edinburgh, where he took a 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), and Simple Heraldry, Cheerfully Illustrated (1953) with Don Pottinger, but his interests also extended to Georgian and Byzantine noble genealogies.
Sir Iain was considered somewhat eccentric (e.g. he had a trick of moving his false teeth up and down while speaking, which he preferred to play on young women). A self-confessed incorrigible 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 letters to newspapers, particularly The Daily Telegraph. He held membership in many London clubs and founded his own club in Edinburgh, called Puffin's, after his first wife, Diana, Countess of Erroll suo jure, whom he married in 1946. They had three children:
That marriage was dissolved in 1964 and in 1966 he took as his second wife Hermione Patricia Faulkner, daughter of Lieutenant-Colonel Walter Douglas Faulkner.