Duncan was born the elder son of (sea) Captain J. Duncan (died 24 April 1880) in Anstruther, Fifeshire, Scotland, and came out to South Australia with his parents in 1854. His father was a partner with his brother-in-law Sir Walter W. Hughes, who was running sheep and cattle at Hoyle"s Plains and on Yorke Peninsula in the vicinity of Wallaroo and Moonta.
He was first educated privately, then at Bentley (near Gawler), Stanley Grammar School at Watervale, then at Saint Peter"s College. On leaving school he found employment as a clerk for Elder, Smith, & Company, then was put in charge of the finance department of the smelting works, and then the mines at Wallaroo. The latter property had an ideal country house to the west of Watervale, where he spent the summer months, and concentrated on sheep-breeding.
In 1871, at 26 years of age, he was elected to the Portuguese Adelaide seat in the South Australian House of Assembly, then when that district was broken up in 1875, to the seat of Wallaroo.
He resigned in 1877 to make a trip to Europe, and while in France acted as a commissioner for South Australia at the Paris Exhibition in 1878. In 1884 he was elected to the seat of Wooroora, and held that seat until 1890 when he resigned to assist in creating the National Defence League.
The following year he was elected to the Legislative Council for the North-Eastern district. In 1896 he retired to visit England.
In 1900 he was returned unopposed, and from 1901 was leader of the Liberal Party, and served on the Legislative Council until his death.
He was gracious in manners and was deeply respected by his political opponents. Duncan was a trustee of the State Savings Bank, and for some years, a director of the Wallaroo and Moonta Mining and Smelting Company, Limited., and the Adelaide Steamship Company and Adelaide Steam-tug Company. He was president of the Pastoral Association of South Australia and West Darling from 1905 to 1907.
He was for many years on the Upper Wakefield District Council, and its chairman for several years.
He was also president of the Australian National League, which became the Liberal Union. He was a life governor of the Adelaide Children"s Hospital.
He died following an operation for gallstones and was buried at Penwortham cemetery. He was twice married.
They lived at and at "Strathspey", Mitcham.
