John Thomas Gannon (1830 – 5 August 1887) was a politician and solicitor in New South Wales, Australia. He was born in Sydney in around 1830, the second child of convict James Gannon and his wife Mary née Phelps. A younger brother Michael would later be a member of the Queensland Legislative Assembly. He attended state schools before becoming an articled clerk to Bob Nichols. On 12 July 1855 he married Harriette Mary Jones, with whom he had four children. He was admitted as a solicitor in 1857. before moving to Goulburn from 1859.