Porritt represented New Zealand at the 1924 Summer Olympics in Paris, France, where he won a bronze medal in the 100 metre dash; the winner was Harold Abrahams (1899–1978). The race took place at 7 pm on 7 July 1924. Abrahams and Porritt dined together at 7 pm on 7 July every year thereafter, until Abrahams' death. The race was later immortalised in the film "Chariots of Fire", but due to Porritt's modesty his name was changed to "Tom Watson". He also won two qualifying races in the 200 m, but did not advance in the semi-final. Porritt was captain of the New Zealand team at the 1928 Summer Olympics in Amsterdam, but withdrew from the 100 m because of an injury.