Denning McTague, who worked at a National Archives and Records Administration facility in the city last summer, pleaded guilty to one federal count of stealing government property.
McTague, of Philadelphia, netted about $30,000 (22,500 EUR) from selling about 150 of the documents online, prosecutors said. All but three of the items have been recovered, prosecutors said.
He told investigators that he took the documents out of the building hidden inside a yellow legal pad.
The stolen Civil War-era documents dating from between 1861 and 1865 included telegrams concerning the troops' weaponry, the Lincoln death announcement sent to soldiers, and a letter from famed cavalryman James Ewell Brown Stuart, prosecutors said.
The investigation began in the fall after a Gettysburg company that publishes books on the Civil War recognized the eBay items as National Archives documents and alerted authorities, officials said.
As an intern, McTague was responsible for arranging and organizing documents in preparation for the upcoming 150th anniversary of the Civil War. His responsibilities included ordnance records dating from 1816 to 1907, prosecutors said.
After McTague entered his guilty plea, U.S. District Judge Stewart Dalzell ordered him released on $25,000 (19,000 EUR) bail and set sentencing for July 12.
The choice of the city of Helsinki is not incidental as the capital of Finland had hosted US-Soviet negotiations on the limitation of nuclear stockpiles in 1969