Mason was educated at Clyde Quay School, then Wellington College where he was dux in 1902. He won a scholarship and attended Victoria University where graduated in 1970 with a Master of Arts with honours in mathematics and an Bachelor of Laws. He then worked in several law firms in Wellington and Eltham before opening his own practice in Pukekohe in 1911. He was soon joined in the practice by his brother Spencer, who later became president of the Auckland District Law Society.