Late in the war, he was promoted to rear admiral and also acted briefly as a brigadier general in the Confederate States Army. His appointment, or arrangement to act, as a temporary brigadier general from April 5 to April 26, 1865 was never submitted to or officially confirmed by the Confederate Senate.
After the Civil War, Semmes faced treason charges by the Union, but these were dismissed in 1866. He became a professor, a judge, a newspaper editor, and an attorney. He also became a staunch "Lost Cause" advocate.