Long, 38, was named in Saints’ Greatest Ever 17, having scored 2,625 points in 330 games for the club, and re-joins on a two-year deal.
Cunningham said: “I am really happy that Sean has agreed to join my team. He is a legend at this club and will bring years of experience not only as a world class player but also as a first rate coach.
“He commands respect amongst the playing group and will complement our backroom team perfectly.”
Long won two World Club Challenges, four Grand Finals and five Challenge Cups as well as being named Lance Todd winner three times.
“I am delighted to be back at a club I love so much,” he said. “Saints are one of the greatest rugby league clubs in the world and to be part of the backroom team here was too good an opportunity to turn down. It was my chance to come home.
“The opportunity to work with Keiron Cunningham – a teammate and friend who was and still is a legend at Saints – and the great squad the club has is very exciting.
“I’d like to thank Salford Red Devils for their support and the opportunity to continue my coaching career there. I wish all the staff and the players the best for the future.
“I’d also like to thank Eamonn and the Board for the opportunity to come back to the club I played the majority of my career for.”