James was the elder son of Alan Moy-Thomas and his wife, Gertrude. He had an older sister. Moy Thomas were coal and builder merchants for many years. In 1914, they had six branches in the London area. James was born was born at Walton-on-the Hill in 1909. The following year, the family moved to 14 Grosvenor Street, W. In 1911, there were six servants including a nursemaid to look after the three children.

He was educated at Eton and Matriculated in 1927 having been awarded a Scholarship in Natural Sciences. His brother, Edward Alfred came up two years after him but remained only one year.

James graduated with a 1st in Zoology in 1930.

In 1933, he married Joy Mitchell at Wharfedale. They had a daughter and a son.

In 1933, he was working a Leeds University.  He was Burdett-Coutts Scholar and returned to Oxford working in the Department of Zoology and Comparative Anatomy having received the Browns Research Fellowship at The Queen’s College. He was a Lecturer at Christ Church. From 1936 his family lived at 9 Holywell Street, where they remained until 1962.

In June 1941, he was granted a commission for the duration, in the Administration and Special Duties Branch of the RAF.

On 29 February 1944, he died as a result of a motor accident.

He is buried in Cambridge City Cemetery Grave 14153.

On 8 April 1944, “Nature” published an obituary
“By the death, in a motor accident while on duty, on February 29, of Flight-Lieut. J. A. Moy-Thomas at the age of thirty-five, we have lost one of the most distinguished of the younger generation of zoologists. The study of fossil fishes attracted him most, and he made many valuable contributions to our knowledge of their structure and classification. Particularly did he devote himself to unravelling the difficult problem of the true affinities of certain well-known forms of very obscure relationship.”

He published many papers on his research.