In 1955, Bearss began working for the National Park Service (NPS) in Vicksburg, Mississippi. He prepared historical studies for the Interior Department agency and founded the Mississippi Civil War Roundtable. For the NPS, he found the Civil War-era cannon "Widow Blakely" (also "Whistling Dick") which had been used in the Vicksburg campaign, as well as the wreck of . He also found two lost forts in Grand Gulf, Mississippi, and was party to "the establishment of Grand Gulf as a state military monument." In 1991, he was made the NPS' chief historian of military sites.