McIntosh was educated at Oriel College Oxford and graduated in 1934. He went on to a diploma in comparative philology at Merton College, and became a student of J.R.R. Tolkien. McIntosh is said to:
- "...one day to have thrashed JRR Tolkien at tennis, confining him to his rooms with an ankle injury. Thus marooned, with nothing better to do, Tolkien started sketching out ideas for The Hobbitt [sic] and The Lord of the Rings works for which Angus would cheerfully accept blame.""
- ― Heinz Giegerich: Angus McIntosh Obituary
As Tolkien, McIntosh was enrolled in the British Amry to decrypt Enigma codes during World War II. At war's end, he returned to a lectureship at Christ Church Oxford, conducting seminars on Middle English with Tolkien, among other things. In 1948, McIntosh moved to the University of Edinburgh, where Tolkien and his daughter Priscilla were to visit him and his wife in July, 1973.