John Harold Mostyn (1887 – 9 July 1956) was the 55th Lord Mayor of Sydney and a rugby league administrator.

Mostyn was elected as an Alderman of the City of Sydney for Camperdown Ward for the Australian Labor Party in 1924.

[2][3][4][5] He left office in early 1928, with the dismissal of the City of Sydney and its replacement by a board of commissioners.

[6][7] Mostyn became the president of the St George District Rugby League Football Club in 1938 and stayed in that role for eight years until 1944, when he lost a ballot for re-election.

[9] A long time resident of Carlton, New South Wales, Jack Mostyn died on 9 July 1956, age 69.