Nordahl Lelandais, 38, had admitted to killing Corporal Arthur Noyer, 23, in the early hours of April 12, 2017, after picking him up as he hitchhiked after leaving a nightclub in the French Alps town of Chambery.
Lelandais has also confessed to killing eight-year-old Maelys de Araujo in August 2017, in a case that horrified France and which is set to go to trial next year.
The prosecution had requested the maximum 30 years in prison for the killing of Noyer, but the court in southwestern Savoie sentenced him to 20 years after finding him guilty on Tuesday.
Lelandais, wearing a white shirt and clasping his hands together, did not react when he heard the verdict, which came after seven hours of deliberation in the high-profile week-long trial.
He has insisted that both deaths were accidental, telling police that Noyer had struck him in a parking lot where they had stopped, prompting a fight that ended when Noyer was knocked out.
Lelandais then put Noyer’s body in the boot of his car.
He then drove around 20 kilometres and dumped the soldier’s body on the side of a road.
During the trial Lelandais offered his “sincere apologies” to the Noyer family, insisting he “never wanted to kill” him.
But prosecutors said the “extreme violence” Lelandais used signalled “the intention to kill”.
Investigators only linked Lelandais to Noyer’s death after he was arrested over the murder of Maelys, who vanished in the early hours of August 27, 2017 while attending a wedding near Chambery with her parents.
Police searched for months for the girl before arresting Lelandais, who was also a guest at the wedding. He finally led them to her remains in February 2018 after traces of her blood were found in his car.
The two cases sparked fears that Lelandais could be involved in dozens of other unsolved disappearances in the region, and investigators reopened several cases after reviewing his background and movements over several years.