In the 1920 Summer Olympics he won a gold medal in middleweight division, defeating Canadian boxer Georges Prud'Homme in the final.

In that year, he met Roger Brousse of France in the quarter-finals, and after the decision came down 2–1 in favour of Brousse, Mallin showed the referee fresh teeth marks on his chest, which further examination proved that Mallin had definitely been bitten by his French opponent.

After the incident versus Brousse, Mallin was referred to by one reporter as "the unroasted human beef of Old England".

[2] Mallin was the first to successfully defend an Olympic title in two consecutive games, and still remains the only male British boxer to do so until Nicola Adams repeated the feat in 2016.

[4] In 1937, he achieved the distinction of being the first British television sports commentator, when he gave commentary on two boxing matches that were broadcast by the BBC from Alexandra Palace.