The Neuruppin Regional Court sentenced him to five years in prison.
The man, who was not identified, had denied working as an SS guard at the camp and aiding and abetting the murder of thousands of prisoners.
In the trial, which opened in October, the man said that he had worked as a farm laborer near Pasewalk in northeastern Germany during the period in question.
However, the court considered it proven that he worked at the camp on the outskirts of Berlin between 1942 and 1945 as an enlisted member of the Nazi Party’s paramilitary wing, the German news agency dpa reported.
“The court has come to the conclusion that, contrary to what you claim, you worked in the concentration camp as a guard for about three years,” presiding Judge Udo Lechtermann said, according to dpa, adding that in doing so, the defendant had assisted in the terror and murder machinery of the Nazis.
“You willingly supported this mass extermination with your activity,” Lechtermann said.
Prosecutors had based their case on documents relating to an SS guard with the man’s name, date and place of birth, as well as other documents.
For organizational reasons, the trial was held in a gymnasium in Brandenburg/Havel, the 101-year-old’s place of residence. The man was only fit to stand trial to a limited extent and was only able to participate in the trial for about two and a half hours each day. The trial was interrupted several times for health reasons and hospital stays.
Sachsenhausen was established in 1936 just north of Berlin as the first new camp after Adolf Hitler gave the SS full control of the Nazi concentration camp system. It was intended to be a model facility and training camp for the labyrinthine network that the Nazis built across Germany, Austria and occupied territories.
More than 200,000 people were held there between 1936 and 1945. Tens of thousands of inmates died of starvation, disease, forced labor and other causes, as well as through medical experiments and systematic SS extermination operations including shootings, hangings and gassing.
Exact numbers on those killed vary, with upper estimates of some 100,000, though scholars suggest figures of 40,000 to 50,000 are likely more accurate.
As in other camps, Jewish prisoners were singled out at Sachsenhausen for particularly harsh treatment, and most who remained alive by 1942 were sent to the Auschwitz death camp.
Sachsenhausen was liberated in April 1945 by the Soviets, who turned it into a brutal camp of their own.
Tuesday’s verdict relies on recent legal precedent in Germany establishing that anyone who helped a Nazi camp function can be prosecuted for accessory to the murders committed there.