In 2022, Irmgard Furchner was handed a two-year suspended sentence for aiding and abetting the murder of 10,505 people during her time as an 18- and 19-year-old secretary at Stutthof concentration camp in the last two years of World War 2.
The German Federal Court rejected her lawyer's argument that her involvement did not go beyond carrying out "everyday" activities as a typist.
Its ruling upholding the lower court's decision is final and cannot be appealed.
"The principle that typical, neutral professional activities of an 'everyday nature' are not criminal does not apply here, since the defendant knew what the main perpetrators were doing and supported them in doing it," judges at the Leipzig court wrote.
Some 65,000 people died of starvation and disease or in the gas chamber at the camp in Stutthof, near Gdansk, at that time still the German city of Danzig.
They included prisoners of war and Jews caught up in the Nazis' extermination campaign.
Many were transported from there to be gassed at Auschwitz.
Furchner's failure to appear for the opening of her trial in 2021 made her, at 96, one of the world's oldest fugitives, leading judges to issue an arrest warrant for her.
She is the latest in a series of nonagenarians to have been charged with Holocaust crimes in what is seen as a rush by prosecutors to seize the final opportunity to enact justice for the victims of some of the worst mass killings in history.
