Mathieson entered an ERA E-Type with Leslie Johnson as driver, for 1949 Richmond Trophy, Jersey Road Race and British Empire Trophy, as well as the 1950 British Grand Prix. Some sources attribute these as official ERA entries because Johnson had purchased the car manufacturer three years before. Mathieson bought a 2-litre Frazer Nash Le Mans in which he scored a class victory in the 1950 24 Hours of Le Mans together with Richard "Dickie" Stoop.