Born in London in 1915, he was the second son of Sir Alan Barlow, son of Thomas Barlow, royal physician. His mother was Lady Nora Barlow, daughter of Sir Horace Darwin. He was a great-grandson of the naturalist Charles Darwin. He was named after his great-great-great-grandfather Erasmus Darwin. His elder brother was Commodore Sir Thomas Erasmus Barlow, a younger brother is Horace Barlow.
He married Brigit Ursula Hope Black (Biddy), daughter of the author Ladbroke Black in 1938. They had three children: