Johann "Hans" Breyer, 89, from Philadelphia, a retired tool-and-die maker, is being held in custody without bail.
He was arrested outside his home hours after a court in Weiden, Germany, issued a warrant charging him with 158 counts of complicity in the commission of murder.
Each count represents a trainload of Nazi prisoners from Hungary, Germany and Czechoslovakia who were killed between May 1944 and October 1944, according to US prosecutors.
Lawyer Dennis Boyle argued at a federal court in Philadelphia that Breyer was too infirm to be detained pending his extradition hearing on August 21.
Breyer, wearing a prison-issue jumpsuit, appeared frail and carried a cane as he was helped to his seat. He had mild dementia and heart issues and has previously suffered strokes.