Prosecutors said Denning McTague, who has master's degrees in history and library science, put about 150 of the documents online and had shipped about half of them.
All but three of the items, worth an estimated $30,000 in all, have since been recovered.
McTague told investigators that he used a yellow legal pad to sneak the documents out while working at the National Archives and Records Administration last summer. As an unpaid intern, he had been responsible for arranging and organizing documents in preparation for the 150th anniversary of the Civil War.
A Gettysburg company that publishes books on the Civil War spotted some of the items on eBay and alerted authorities last fall, officials said.
McTague pleaded guilty to one federal count of stealing government property.