Mihm was an assistant prosecuting attorney for St. Louis County, Missouri from 1967 to 1968; assistant state's attorney for Peoria County, Illinois from 1968 to 1969; and assistant corporation counsel for the City of Peoria from 1969 to 1972. He was State's Attorney of Peoria County from 1972 to 1980, then in private law practice in Peoria from 1980 until becoming a federal judge in 1982.[3]
Mihm presided over the case of Ali Saleh Kahlah al-Marri, who pleaded guilty in 2009 to conspiring to provide material aid to Al Qaeda.[4] Federal authorities arrested al-Marri in 2001 for credit card fraud, then deemed him an enemy combatant, and detained him without charge for over six years. The federal government claimed that al-Marri was an al Qaeda sleeper agent. The Court of Appeals for the Fourth Circuit ordered al-Marri brought before a civilian court in 2007,[5] and in 2009, the United States dropped its appeal of that decision, and transferred al-Marri to Mihm's court.[4]