McCulloch was appointed U.S. marshal for the Eastern District of Texas in 1852, serving throughout 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 two Peace Commissioners sent to deliver President Buchanan's pardon to Brigham Young in Utah (the other being former Gov. Lazarus W. Powell of Kentucky). As the Commissioners stated during meetings with Brigham and Church leadership, they had no power to negotiate. The Buchanan pardon was non-negotiable: 1) submit to Federal authority, and 2) allow the army to pass through Salt Lake City and establish a post somewhere in Utah. Brigham accepted the pardon under protest that the members of the Church had committed no crimes other than burning the US Army supply train contracted with Majors & Waddell, on the high plains of Wyoming. McCulloch in effect ended the Utah War when he changed the tone of the 2-day meeting on the 2nd morning, June 12, 1858, as reported by the New York Herald, 9 August, 1858, 8/2-3: "But while a torrent of beastly and disgusting words were issuing from the throat of Erastus Snow, Commissioner McCulloch interrupted him with this remark: "...but I tell you sir, the army shall come in, and no power here can prevent it.""