Armstrong was born in Newcastle upon Tyne. He was a surgical sublieutenant in the Great War. Forestalled in his desire to become a neurologist, he became a general physician and then an endocrinologist. From 1931 he was a consultant physician to the Royal Victoria Infirmary in Newcastle.
He was a founder member of the endocrinology section at the Royal Society of Medicine. He specialized in intersex patients, and gave lectures on the subject in Britain and abroad.
His criteria re disputed sex were used by several judges. His advice that there are four criteria, vis chomosomal, gonadal, phenotypical and psychological, was accepted in the Ewan Forbes case. In Corbett v. Corbett he argued that April Ashley was a pre-birth intersex and should be assigned to the female sex, but his advice did not prevail. In 1979 Mark Rees was invited for an evaluation possibly leading to phalloplasty, but the surgeon retired and was not replaced.
- Charles Nathaniel Armstrong, and Alan John Marshall. Intersexuality in vertebrates including man. London: Academic Press. 479 pp 1964.
Mark Rees. Dear Sir or Madam: the autobiography of a female-to-male transsexual. London & New York: Cassell, 1996: 147-151.
- “Charles Nathaniel (‘Natty’) Armstrong”. British Medical Journal. 27 Feb 1999.