Mignot is the 11th player to leave Auxerre this summer as new manager Laurent Fournier completely revamps his squad.
The 30-year-old defender has spent his entire career at the Burgundy club but is excited about the prospect of a new challenge further south.
Mignot said of his move: "It's pretty moving. I've left the only club I've ever played for to join another club that is just as much like a big family.
"I'm really excited to start the new season with ASSE. You really get the feeling when you arrive here that it's a club with a huge history, and it was the first time in my career that a coach called me directly to express his interest in my signature.
"That really counted for something and then the plans that [St-Etienne manager] Christophe Galtier described to me were very convincing."
Galtier said: "We've been following Jean-Pascal very closely for two or three seasons.
"We were already looking at signing him when he extended his deal at Auxerre. His ability to perform consistently is a real asset; he has a wealth of experience in Ligue 1 and has also played in the Champions League, like many of our recruits this season.
"He's very ambitious in terms of results and quality of play.
"In athletic terms, he is ready to go, he has done a full pre-season with Auxerre, and he played in all the friendly games. As of Tuesday, we'll start working on integrating him into our tactical plans."