Joseph Martin and Christopher Heikkila were employed at the Army & Air Force Exchange Service, which operates stores and entertainment at US bases, and living in Germany while their parents were civilian employees of the military, according to the US Department of Justice.
Martin, 20, now resides in Arizona, where the trial took place. Heikkila, 21, still lives near Ramstein, the sprawling European and African headquarters of the US Air Force.
The assault occurred in Landstuhl, outside Ramstein, while the girl was incapacitated, the DOJ said.
The men had exchanged social media messages planning and, afterward, commenting on the sexual assault, evidence in the case showed.
US federal courts have jurisdiction over felonies committed overseas by those employed by or accompanying the US military.
The pair were each convicted of one count of sexual abuse and one count of abusive sexual contact.
Sentencing is scheduled for November.