Lucien Leger, 68, spent his first night of freedom northern France at the house of a butcher, who was a long-time friend.
"I feel like someone who is no longer in prison, but exactly as if I were still in prison," Leger told LCI TV after his release.
He reportedly celebrated his release by drinking champagne with friends and supporters.
Nicknamed "the strangler," Leger was incarcerated in 1964 for having killed 11-year-old Luc Taron. Two years later, a court sentenced him to life in prison.
Leger initially confessed to the killing, but later retracted his confession. The victim's mother, Suzanne Taron, said she regretted the court's decision.
"It's sad, but I think it had already been decided," she told France 2 TV, "and had I opposed it, I don't think anyone would have listened to me."
During his four decades in prison, Leger petitioned for his release on more than a dozen occasions. He was finally granted his wish last August.