Samuel Bland Arnold (born 1838 in Washington, DC) was also involved in the plot to kidnap Pres. Lincoln, and appears not to have had much, if anything, to do with his murder. Perhaps a close look at the transcripts of his trial will reveal more about him - some of these are available on Footnote. He was sentenced to life imprisonment (on the charge of conspiracy to murder) at Fort Jefferson on the Dry Tortugas with O'Laughlen, Dr. Mudd and Edmund Spangler, and spent much of his time there as a clerk at headquarters. He was pardoned and released in 1869. He was accompanied home by his father and fellow-prisoner Edmund Spangler. The Baltimore Sun of 7 April 1869 stated that "Arnold appears in rather delicate health". In 1898 he returned to the prison to take some photos but these have apparently disappeared. He lived quietly and died in 1906; he is buried in Baltimore.
Samuel's father, George, was a baker in Baltimore in the 1850, 1860 and 1870 censuses. The family seems to have been fairly well-to-do since they had a number of servants and apprentices living with them. Aged 15 in 1850, Samuel was living with his parents, and again in 1870 he was with them, but not in 1860. His occupation in 1870 was listed as a farmer; by 1880 he was age 67, boarding with a widow in Anne Arundel County, MD, and listed as a manager (presumably of her farm). Other than the fact that Arnold wrote a series of newspaper articles for the Baltimore American in 1902 describing his imprisonment, not much else is known about him.