 On January 13, 1866, President Andrew Johnson nominated McCook for appointment to the grade of brevet brigadier general of volunteers, to rank from March 13, 1865, and the United States Senate confirmed the appointment on March 12, 1866.McCook returned to Steubenville following the war and was admitted to the bar in 1866.He served as the U.S. assessor of internal revenue taxes from November 1865 until May 1873, when he moved to New York City, established a law practice, and entered politics.