Duncan Stewart was born at Glenlyon, Perthshire, Scotland, in 1834. His father died when he was a young child, and he migrated to Australia with his mother in 1838, landing at Port Phillip, Victoria. His mother married James Smith in Melbourne and in 1846 the family settled at Rivoli Bay, South Australia, where they operated a store and a tannery. In 1854 the family moved to Mount Gambier.

Stewart and his mother, Christina Smith, were actively involved with Aboriginal people in the south-east of South Australia. Stewart was first appointed as an Aboriginal interpreter in 1848, and appears to have held this position until 1857. Christina Smith established an 'Aborigines Home' in Mount Gambier in 1865, and in 1880 published The Booandik Tribe of South Australian Aborigines (Government Printer, Adelaide). Stewart's interest in the 'Buandik' people is reflected in his notes and observations later collected as 'Aborigines of the Buandik Tribe of the South East of South Australia'. Stewart also contributed to ethnographic works published in the 1880s, including a chapter on Mount Gambier and a 'Booandik' vocabulary in Edward M Curr, 1887, The Australian Race, Melbourne, John Ferres.

Stewart spent most of his adult life working on stations in the south-east of South Australia, including as an overseer at Glencoe Station, and also spent some time as a mail contractor. In 1879 he took up land at Rendlesham, South Australia, and spent the rest of his life there. He died on 27 August 1913, leaving a widow and six children.

Note that the State Library of South Australia and State Records of South Australia also hold material relating to Duncan Stewart.