Charles de Monchaux was educated at St Joseph's College, entering the school in 1913 as one of the first ten state bursars on the introduction of the government bursary scheme that year. In the Leaving Certificate he was fourth in the year, winning a University bursary and exhibition, and the Freehill scholarship to St John's College, University of Sydney.

He graduated in 1922, was a resident at Royal Prince Alfred Hospital and spent three years in the Health Department, Brisbane. In 1926 he went into private practice and was appointed an honorary assistant physician at the Mater Hospital, Brisbane. In 1929 he went to London where he studied radiology at Cambridge, and was awarded a Diploma in Medical Radiology and Electrology.

He returned to Brisbane in 1930 to resume private practice. In 1931 he was appointed honorary radiotherapist to the Dunedin Hospital, and lecturer in radiology at the University of Otago. He was in charge of the x-ray and radium department for five years. In 1934 he was awarded a Rockefeller travelling fellowship, visiting the leading radiological departments and radium institutes in North America, Europe, and Great Britain. He returned to Dunedin in 1935, and in 1936 moved to Sydney.

He was honorary radiotherapist at St Vincent's Hospital, Sydney from 1936 to 1960, and honorary consultant radiotherapist from 1960 till 1972. From 1937 to 1967 he was honorary radiotherapist at Lewisham Hospital, and registrar of the Hallstrom Tumour Foundation at the Royal Prince Alfred Hospital during 1952-53. He was honorary consultant radiotherapist at the Mater Hospital, Crows Nest 1955, and at Prince Henry Hospital 1961.

Charles de Monchaux was an eloquent public speaker, an accomplished musician, and a linguist, fluent in French, German, and Italian. He was a life-long student of Latin and Greek and the inventor of an international language, Kosmik. He was a member of the council of St John's College, University of Sydney, and a member of the Australian Catholic Historical Society. He twice unsuccessfully contested seats as an endorsed Liberal candidate at federal elections.

He married Clare Dooley who survives him. His two sisters live in retirement in the country. His nephew Robert J de Monchaux is a Fellow of the College.