Millin graduated in medicine in 1927 from Trinity College Dublin after also gaining a degree in maths and arts, and representing both his university and Ireland at rugby.

He first became a house surgeon at Sir Patrick Dun's Hospital, Dublin, following which he gained postgraduate qualifications and moved to London with a travelling scholarship.

His three-page article on the retropubic prostatectomy, published in The Lancet on 1 December 1945, demonstrated a method of removing the prostate without the traditional cut through the bladder, thus reducing complications, and he became renowned for the procedure.

Related to Sir James Pitcairn, surgeon and descended from the ancient Pitcairn family of that Ilk, Fifeshire and a cousin of Edward John Chalmers Morton of Frocester Court, Glos, MP for Davenport,[citation needed] his father was a successful barrister and honorary librarian of the Statistical and Social Inquiry Society of Ireland.

[1][3] After completing school, he gained admission to Trinity College Dublin in 1921, with a scholarship to study arts and mathematics.

[3] In 1930, he became a Fellow of the Royal College of Surgeons of England and the following year took the optional subject of genito-urinary surgery in his MCh examination at Dublin, when he came first place.

A new extravesical technique: report on twenty cases”, a procedure he had demonstrated at the French Urological Society in Paris in the preceding October.

[3] His social circle included the novelist Elizabeth Bowen, writer Edward, 5th Baron Sackville, The Slazengers of Powerscourt and Stephen and Lady Ursula Vernon of Bruree, the latter a daughter of 'Bendor', 2nd Duke of Westminster.