Sims was born in Georgia to James Sims and Minda Campbell, both slaves under rice planter James Potter. The exact date of his birth is unknown, but it is estimated to be around 1828. He had several brothers and sisters, including James M. Simms, another notable African American of the time. Before his escape, Thomas Sims worked as a bricklayer for Potter. During this time he also married and had children with a free African American woman.
