After this he served as a judge advocate and aide to his wife's cousin, Lt. Gen. James Longstreet. Later in 1862, Confederate States President Jefferson Davis appointed Lamar as Confederate minister to Russia and special envoy to the United Kingdom and France. When the Civil War was over, he returned to the University of Mississippi where he was a professor of metaphysics, social science and law. In 1865, 1868, 1875, 1877, and 1881, he was also a member of Mississippi's constitutional conventions.