Prosecutors said 83-year-old Josias Kumpf served as a guard at the Sachsenhausen concentration camp in Germany and the Trawniki labor camp in Nazi-occupied Poland, and at slave labor sites in occupied France.
U.S. investigators found he participated in a 1943 mass shooting in Poland in which 8,000 Jewish men, women and children were murdered in pits at Trawniki in a single day.
"Josias Kumpf, by his own admission, stood guard with orders to shoot any surviving prisoners who attempted to escape an SS massacre that left thousands of Jews dead," Acting Assistant Attorney General Rita Glavin said in a statement.
Investigators say Kumpf joined the SS Death's Head guard forces in 1942.
Kumpf was born in what is now Serbia, immigrated to the U.S. from Austria in 1956, settled in Racine, Wis., and became a U.S. citizen in 1964.
A federal judge in Milwaukee had previously found Kumpf did not disclose he had been an SS guard because he feared it would disqualify him when he applied for a visa to the U.S.
In a 2003 interview, Kumpf said he was taken from his home in Yugoslavia as a 17-year-old and forced to serve as a guard, but he didn't participate in any atrocities.
Since 1979, the U.S. Justice Department has won cases against 107 people who participated in Nazi crimes.