| Jack Mostyn | |
| Office: | 55th Lord Mayor of Sydney |
| Term Start: | 1 January 1927 |
| Term End: | 3 January 1928 |
| Deputy: | Frank Green |
| Predecessor: | Patrick Vincent Stokes |
| Successor: | Council dismissed |
| Office2: | Alderman of the City of Sydney for Camperdown Ward |
| Term Start2: | 1 December 1924 |
| Term End2: | 3 January 1928 |
| Alongside2: | Ernest Charles O'Dea |
| Predecessor2: | Charles Mallett David Gilpin |
| Successor2: | Council dismissed |
| Birth Date: | 1887 1, df=yes |
| Birth Place: | Orange, Colony of New South Wales |
| Death Place: | Rockdale, New South Wales, Australia |
| Resting Place: | Woronora Memorial Park |
| Spouse: | Laura Champley (b.1887–m.1910–d.1972) |
| Party: | Australian Labor Party (NSW Branch) |
John Harold Mostyn (1887 – 9 July 1956) was the 55th Lord Mayor of Sydney and a rugby league administrator.
Jack Mostyn was born at Orange, New South Wales in 1887 and was an electrician by trade.
Mostyn was elected as an Alderman of the City of Sydney for Camperdown Ward for the Australian Labor Party in 1924.[1] In 1927 he became Lord Mayor of Sydney.[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.[8] He was president during the club's first premiership win in 1941. Mostyn was also the St. George Dragons delegate to the NSWRFL for many years.[9]
A long time resident of Carlton, New South Wales, Jack Mostyn died on 9 July 1956, age 69. He was later buried at Woronora Cemetery, Sutherland, New South Wales.[10]