Marshall, 43, who left Halifax two months ago, has signed a deal with the Betfred Super League champions to the end of the 2020 season.
Long left the club earlier this week to take up a position in rugby union at Harlequins.
Marshall was assistant to Warrington coach Tony Smith before joining Halifax and guided the part-time Championship club to three top-four finishes in his four seasons at The Shay.
Coincidentally, he will go up against his former club when Saints play Halifax in the Coral Challenge Cup semi-final at Bolton on July 27.
Marshall, who was a player with Halifax, Huddersfield, London Broncos and Leigh, said: “I had a number of options after I left Halifax, but when a club like St Helens comes calling, you jump at the chance.
“This is a wonderful opportunity to continue my development at a fantastic club and I cannot wait to get started.”
St Helens chief executive Mike Rush said: “We all welcome Richard to the club with open arms. I have known him for over 15 years having seen him rise through the ranks at Warrington and more recently at Halifax.
“He absolutely fits the culture of the club. He is hard-working, driven and a talented individual who will bring further experience to our coaching staff.”