The public prosecutor in the canton of Aargau found the politician guilty of driving while incapacitated and causing bodily harm through negligent driving in a judgement published on Monday.
Müller, who was campaigning for a seat in the Senate at the time, was driving his Mercedes coupe towards Lenzburg on September 10th when the car drifted across the central line and collided head-on with a scooter coming in the opposite direction.
The 17-year-old driver of the scooter suffered serious injuries, including fractures to both legs.
The investigation concluded that Müller's car drifted across the road because he was fatigued due to sleep apnea, the prosecutor said in a statement.
However the politician should have known he was tired because his vehicle's brake-assist function and lane departure warning had been activated several times on the journey, it said.
Prosecutors arrived at that conclusion after monitoring images from the vehicle's dashboard camera.
As a result of the crash Müller, 63 at the time, suspended his election campaign “out of respect” for the victim.
He was also forced to defend his behaviour following the accident, which was criticized by some media outlets.
Speaking to Blick at the time, the father of the victim said Müller did not stop immediately after the accident, but 300 metres down the road.
When he did return to the scene, he didn't try to speak with the girl's father, who was by then present at the scene.
In a statement, Müller said “After the accident I was in a state of shock and therefore I stopped my vehicle only at the next possible parking spot and immediately returned to the site of the accident.
“I didn't know that the father of the young woman was there. I didn't know him and the police didn't tell me that one of the young woman's parents was there.
“Naturally I would have sought to speak with him.”
Müller, who was duly elected to the Senate, was succeeded as president of the FDP by Petra Gössi in in April 2016.