After working in iron foundries in England, Seddon went to Australia in 1863 to work at the Bendigo goldfields in Victoria. He moved to Hokitika, N.Z., in 1866, again for gold mining, and in 1869 he became an advocate for miners in goldfield disputes. His prominence in local politics gained him a seat in Parliament in 1879. Serving as minister of mines and public works in the Liberal ministry of John Ballance (1891–93), he abolished subletting of government contracts for public works. He succeeded Ballance in 1893, inheriting a bill for woman suffrage, which was passed the same year, and also a talented cabinet, including William Pember Reeves (Reeves, William Pember) and John McKenzie. (McKenzie, Sir John)

Under Seddon's leadership, Reeves's influential Industrial Conciliation and Arbitration Act (1894) and McKenzie's land act to assist small farmers (1894) were enacted. The Old Age Pensions Act of 1898 is considered Seddon's greatest legislative achievement. An imperialist in foreign policy, his attempt to incorporate Fiji into New Zealand failed, but he successfully annexed the Cook Islands (1901). He also bought vast amounts of land from the native Maoris and was opposed to Oriental immigration. Ardently pro-British, he supported England with troops in the South African War (1899–1902) and sponsored preferential tariffs for trade with the mother country.

Seddon assumed many of the cabinet positions himself after Reeves (1896) and McKenzie (1899) resigned. He died suddenly while returning from Australia shortly after his fifth consecutive national electoral victory.