In May 1861, Union officials began arresting Marylanders suspected of being southern sympathizers. Many were never charged with crimes and never received trials. Others were released after pledging not to "render any aid or comfort to the enemies of the Union," or by taking an oath of allegiance. During the Civil War, Fort McHenry also served as a Union transfer prison camp for Confederate prisoners of war. Prisoners were usually confined for short periods before transfer to larger prisons such as Point Lookout, Fort Delaware, or Johnson's Island.
Life at Fort McHenry was difficult. Each prisoner was given 1 blanket, but was denied bedding, chairs, stools, wash basins, and eating utensils. They used tin cups, pocket knives, forks and spoons whittled from bits of wood, and hardtack for a plate. Prisoners received 3 meals a day: breakfast, consisting of coffee and hardtack; a second meal of bean soup and hardtack; and a main meal of coffee, 1/2 lb. of salt pork or pickled beef, and hardtack. Occasionally, the meat was rancid and the hardtack moldy. Prisoners who could afford to bought fresh fruit, vegetables, and comfort items from sutlers. Also, sympathizers in Baltimore sent food, clothing, blankets, and money to supplement available resources.
The prisoners spent their time in a number of activities. They formed literary societies and debating teams. Many made trinkets which they traded for extra rations. There were daily ball games and rat hunts. Some evenings, the prisoners even staged shows.
The prisoners at Fort McHenry came from all classes and walks. Among the most prominent civilians detained at the fort were the marshal of the Baltimore City Police Force and the Board of Police Commissioners, Baltimore's mayor, a former Maryland governor, members of the House of Delegates from Baltimore City and County, the 4th Congressional District congressman, a state senator, newspaper editors, and other influential citizens. Military prisoners included privates, officers, chaplains, and surgeons.
At times, the inmate population strained the prison facilities. In February 1862, the fort held only 126 prisoners, which swelled to 800 inmates by early 1863. In July 1863, following the Battle of Gettysburg, there were 6,957 prisoners. Once this large influx was dispersed, the monthly population ranged from 250 to 350 prisoners. During the last months of the war, the number dwindled sharply until, by September 1865, only four prisoners were being held.
In contrast to the high death tolls at other prisons, only 15 deaths were recorded at Fort McHenry. At least 3 men were executed at the Fort, to include a Union soldier hanged for the murder of an officer; another shot for desertion and the attempted murder of several civilians; and a Confederate sympathizer found guilty of murdering two civilians while practicing guerilla warfare.
Because of its role as a prison camp during the Civil War, Fort McHenry became known as the "Baltimore Bastille."