In 1778, at age 21, he enlisted in the Illinois Militia and participated in General George Rodgers Clark's Illinois campaign serving as a lieutenant and quartermaster under his brother Major Joseph Bowman. During this time, he was assigned to escort a number of high-level British officials and military officers as prisoners-of-war from Fort Vincennes to Williamsburg, Virginia, including Governor Henry Hamilton and Philippe-François de Rocheblave. He also delivered messages, including letters from his brother Joseph describing the progress of the expedition. Returning to Illinois, he was reportedly present at the capture of Fort Vincennes and attended the burial of his brother in August 1779. He also paid the expense of the services. He was one of the officers awarded a land allotment in Clark's Grant, Bowman being given for his services. Part of his land was used to build Jeffersonville, Indiana, in 1802, the city eventually becoming the county seat of Clark County.