T homas Duff, along with his brother James Duff, is the co-owner of Duff Capital Investors, a massive and highly diversified privately held conglomerate based in Mississippi. He and his brother inherited their father's small tire business, Southern Tire Mart, and have since built it into the largest commercial truck tire dealer in the United States. The success of this foundational business provided the capital for their massive expansion and diversification.

Through Duff Capital Investors, the brothers have built a sprawling empire with interests in a wide range of industries. Their major holdings include the construction company T.L. Wallace Construction, the energy firm KLLM Transport Services, and the poultry producer Southern Hens. They are known for their quiet but highly effective management style, and for building one of the largest and most powerful private companies in the American South. They are also major philanthropists in their home state of Mississippi.