Kay Masterman was the eldest son and one of six children of Charles Edward Masterman, civil engineer, and his wife Lilla née Osmond who lived in Northwood, Middlesex, England and then Westerham, Kent. He was the brother of Nan Chauncy (q.v.). In 1912 Charles Masterman suffered a business setback and took the family to Tasmania where he was the Hobart City Engineer. In 1914 the family moved to Bagdad north of Hobart and grew apples on their property. Kay Masterman built a small stone and concrete cottage on his Bagdad property and gave it to his sister, Nan, as a wedding present.
Masterman was awarded a Master of Arts and in 1927 was Acting Professor of Classics at the University of Tasmania. He was for many years Classics master at Geelong Grammar School in Victoria (possibly from the late 1920s and certainly in 1931). From 1955 to 1960 he was Associate Professor of Classics at the Australian National University, and Head of Classics in the Canberra University College in 1956. He went on to teach at Canberra Boys' Grammar School. In 1947 he published A Latin Word-List for Use in Schools.