In 1907, Thompson Brothers' Lumber Company built a mill and 37 acre pond at Trinity. Texas Long Leaf Lumber Company, which started at Willard in 1912, bought Thompson Brothers' Trinity mill in 1922. Under management of Paul Sanderson for 22 years, the company was one of the nation's most successful manufacturers of Yellow Pine and Southern Hardwoods. Working with forest landowners in Texas, Sanderson promoted selected cutting, reforestation, fire protection and removal of defective trees. By 1937 the mill was producing 3.5 million board feet monthly. The commercial species of lumber cut were Long Leaf, Short Leaf, and Loblolly Pine, Oak, Gum, Ash, Magnolia, and various other woods.

The main office of Texas Long Leaf Lumber Company was the 205 acre mill site in Trinity, with additional sales offices in Houston and St. Louis. The company was one of Trinity's largest employers, with as many as 450 employees. During World War II, production reached 140,000 board feet daily, resulting in lumber flown to Europe and Africa and the awarding of the Army-Navy "E" Award in a March 17, 1944, ceremony in Trinity. Sanderson turned to a widespread public campaign for postwar planning and perpetuation of the timber industry in Texas with emphasis on timber as a renewable natural resource. At the time of his untimely death in October 1944, Sanderson held many Civic and Lumber Industry Offices, including President of the Southern Pine Association, President of the Texas Forest Association and Chairman of the Texas Prison Board. In 1953, Texas Long Leaf Timber interests totaled 252,230 acres in six East Texas counties. The closing of the mill in 1955 had a devastating impact on the local economy and ended a long chapter in the history of the community.

The base of the Historical Marker includes bricks from "The Old Mill".