William Henry Mahony (9 February 1856 – 26 July 1918) was an Australian politician. He was born in Adelaide to builder Timothy Mahony and Elizabeth Johns.
He was educated at Sydney Grammar School and was articled as a solicitor's clerk to Robert Burdett Smith in 1877.
In 1882 he was admitted to practice as a solicitor.
In 1894 he was elected to the New South Wales Legislative Assembly as the Free Trade member for Annandale.
He held his seat until 1910, when he retired from politics.