He studied at Bridgnorth Grammar School in Shropshire, and then at Jesus College, Oxford, where he was a scholar.
His university studies were interrupted by the Second World War: he won his scholarship in 1937, but did not obtain his first-class degree in history until 1948.
During the war, he served with the Manchester Regiment and then in the Indian Army, reaching the rank of Major and working in the Mechanical Transport Training Centre as well as interpreting Urdu.
[1][3] His particular field of interest was the Norman Conquest, and one of his articles on Roger of Montgomery (the first Earl of Shrewsbury) and his sons, written in 1963, was described in his obituary in The Times as "indispensable for understanding how the Norman Conquest worked on the ground", particularly in Mason's home county of Shropshire.
[1] He also wrote a history of Bridgnorth and co-authored (with E. G. W. Bill) Christ Church and Reform, 1850–1867, looking at the changes in Oxford during the 19th century.