Theriot has studied freshwater and marine biology for the large part of his career, focusing on modern lakes and on paleontology of lakes. He mainly studies microscopic algae known as diatoms, which leave a dense fossil record in lakes and oceans. His research has two parts: the taxonomy of diatoms, and how they interact with each other and the environment. He is currently working on a study of diatoms in Yellowstone Lake and other lakes in Yellowstone Park, which is helping to reveal how El Nino events affect precipitation and fires in Yellowstone National Park. His work has taken him all over the world, including China, Canada, France, Sweden, Finland, Mexico, Guatemala, and Costa Rica. Theriot was a curator of diatoms at the Academy of Natural Sciences of Philadelphia for eight years before coming to Texas to direct the Texas Memorial Museum. During his last three years in Philadelphia, he also served as the vice president for research and collections. He is also Past-President of the Board of Directors of the Association of Systematics Collections, an organization representing natural history museums in Washington, D.C. Theriot is currently beginning a survey of the diatoms and algae of the lakes of the so-called Highland Lakes (a chain of man-made reservoirs in the Edwards Plateau) and surrounding areas in Central Texas. He is the director of the Texas Natural Science Center.