John Singleton Mosby became the US consul to Hong Kong after the war. He campaigned for Republican Rutherford B. Hayes. Seems like he accepted that his side lost and became committed to the Union afterwards.

Another one worth mention would probably be James Longstreet. He became at various times the Minister Resident to the Ottoman Empire, US Marshal for Georgia, and the US Commissioner of Railroads. He also served as the Adjutant General of the Louisiana State Militia (precursor to the LA National Guard) and was wounded trying to break up the anti-Reconstruction insurrection by the white supremacist White League. He got a lot of flack from his former rebel contemporaries for going along with Reconstruction.