McIlwaine was the only daughter and eldest of three children of Rev. Michael Herbert Thorold (1904–1956),
MA, vicar of St John the Baptist,
Bathwick,
Somerset, and Dorothy, daughter of George Patrick Henfrey, of Chesham Place,
Brighton,
East Sussex. Her father descended from the book collector and politician
Sir John Thorold, 9th Baronet through his second son, and was from a line of several generations of clergymen. Her name, "Ia", was of Cornish origin, deriving from Saint
Ia of Cornwall, and etymologically linked to the place name
St Ives, Cornwall. McIlwaine was a pupil at
Bath High School before reading Classics at
Bedford College, London. In 1958 she gained a Graduate Diploma in Librarianship from University College London. ==Career==