Mengistu ruled the East African nation between 1977 and 1991 following the ouster of Emperor Haile Selassie in 1974.
Alemu – an aide to Mengistu Haile Mariam – had dual Ethiopian-Dutch citizenship.
He denied all the charges leveled against him.
More than 300 victims were named in four war crimes charges.
Ethiopia has already sentenced him to death in absentia.
Prosecutors said that Alemu had been the henchman for Mengistu in the north-west Gojjam province.
The case was tried under Dutch universal jurisdiction laws.
Presiding judge Mariette Renckens told the court that Alemu was “guilty of war crimes and treated his fellow citizens in a cold and calculating manner… including robbing them of their right to life”.
Families of victims applauded the sentence, but neither Alemu nor his lawyers were present in court.