The two women, aged 37 and 17, were attacked after visiting a festival in the Bavarian town of Erding, northeast of Munich, police reported.
As they walked to their car at around 12.30am, a 23-year-old man launched himself at the mother and pulled her to the ground.
The 37-year-old fought to escape, while her daughter struck him with her handbag.
The man then forced his tongue into the mother's mouth – but biting down hard, she managed to sever part of the tongue.
Severely injured, the man fled.
The women alerted police, and officers were later able to arrest the bloodied man in the hospital, where he had been taken after collapsing when he returned to the festival.
Escaping with a few scratches, the both women were in shock after the incident, police said on Tuesday.
The attacker was treated in hospital before being taken into custody, and an investigation into the attack is currently underway.