Immediately after he had returned from the war, Douglas ran into trouble with the United States military authorities. He was arrested and sentenced to three months in prison, because he had been seen in public in his Confederate uniform. Douglas did not serve the full term of the sentence, but was dragged into the investigation after the assassination of President Lincoln, taken to Washington as a witness before the military commission that tried the assassins. After his release in Washington he was arrested, and discharged several times, before taken to Fort Delaware where he after some time in confinement was set free by military orders. Douglas practiced law, at first in Winchester, but after two years, he moved his law office to Hagerstown, where he remained for the rest of his life. Douglas was a successful lawyer and an effective speaker. He ran for Congress in 1886, and was leader of the Gold Democrats in Maryland 1896. He was appointed associate justice of the Maryland Court of Appeals by Governor Jackson, but was not confirmed in the judicial elections in November the same year. Douglas kept his military interest alive; in 1876 appointed colonel on the staff of Governor Carrol. During the railway strike of 1877, Douglas commanded the Maryland forces in the Western Department of the state. His commission expiring with the outgoing governor, Douglas in 1880 became captain of a Hagerstown militia company (Hagerstown Light Infantry), and the following year lieutenant colonel of the 1st Infantry Battalion, Maryland National Guard, and colonel of the 1st Infantry Regiment, Maryland National Guard 1886 until 1891. Douglas was appointed adjutant general of Maryland 1892, and served under Governor Brown's term, until 1896. During the Spanish–American War, he was offered a commission as brigadier general of volunteers, but had to decline due to ill health. Douglas died in Hagerstown of tuberculosis 1903