McCulloch was appointed U.S. marshal for the Eastern District of Texas in 1852. He served during the Pierce and Buchanan administrations. However, conscious of his lack of formal military education, he actually spent much of his term studying military science in libraries in Washington, D.C. In the aftermath of the Utah War, in 1858 he was one of the peace commissioners sent to negotiate with Brigham Young in Utah (the other being former Gov. Lazarus W. Powell of Kentucky).