Augustus sent his new XXI legion to Hispania Tarraconensis to fight the campaign against the Cantabrians. In 15 AD, the legion was stationed in Regensburg, in the new province of Raetia. After the disaster of the Battle of the Teutoburg Forest, the legion was sent as reinforcements to Germania Inferior, where they shared the base camp of Xanten with the V Alaudae. In 43 AD, they were relocated in Vindonissa (Windisch) in the province of Germania Superior.
Along with the rest of the German border army, the XXI Rapax supported their commander, Vitellius, in the year of the four emperors (69 AD) and marched to besiege Rome. Vitellius was, however, defeated by Vespasian before the end of the year.
In 70 AD, the legion took part of the army sent to deal with the Batavian rebellion and relieve the four legions imprisoned by Civilis. After that they were sent to Germania Superior, where they shared the Mainz (Moguntiacum) base camp with Legio XIV Gemina.
In 89 AD, the Mainz legions supported their commander, Lucius Antonius Saturninus, in his revolt against emperor Domitian. After the end of this harmless insurrection, the legions were separated and XXI Rapax sent to Pannonia. It was in this province that they were destroyed during the revolt of the Sarmatians in 92 AD.
See also: List of Roman legions