In 1864 he went to Russia, where he served as secretary to Cassius M. Clay, Minister to the Russian court. During his time in Russia, Curtin became friends with Konstantin Pobedonostsev, professor of law at Moscow State University. He also visited Czechoslovakia and the Caucasus, and studied Slavic languages. While continuing to improve his Russian language skills, he also studied Czech, Polish, Bohemian, Lithuanian, Latvian, Hungarian, and Turkish. Curtin returned to the United States in 1868 for a brief visit. Clay assumed that around this time Curtin made some comments to William H. Seward that cost Clay an appointment as Secretary of War. Clay referred to Curtin as a "Jesuit Irishman".