He owned 15 cotton and sugar plantations, served as President of the Bank of Mississippi, and held major investments in railroads and lumber.

[1] In the 1830s, Duncan was one of the co-founders of the Mississippi Colonization Society and helped purchase land in West Africa, known as Mississippi-in-Africa, to create a colony for relocation of free people of color from the state.

His family were early settlers to the Cumberland Valley in Pennsylvania and his grandfather received a land grant from King George III of Great Britain.

[1] He was a partial owner of the Erie & Kalamazoo, Columbus, Pequa & Indiana, Terre Haute & Richmond and Panama railroads.

[2][15] By 1860, Duncan's ownership of 858 slaves in Issaquena County made him second nationally to the estate of Joshua John Ward of South Carolina, which enslaved 1,130.

[16] While Duncan enjoyed the Mississippi weather during the winter months, he spent most summers away from Natchez and escaped the heat with his family to Philadelphia, Saratoga Springs, New York or Newport, Rhode Island.

[18] In the 1830s, he co-founded the Mississippi Colonization Society along with major slave owners Isaac Ross, Edward McGehee, John Ker, and educator Jemeriah Chamberlain, president of Oakland College.

Their goal was to relocate free blacks and newly freed slaves to the developing colony of Mississippi-in-Africa in West Africa.

[18] Duncan died on January 29, 1867, in New York City, and was interred in Laurel Hill Cemetery in Philadelphia, Pennsylvania.