The 36-year-old former St Helens favourite, who has been on the City Reds' coaching staff since retiring as a player in the autumn of 2011, handed in his notice on Thursday and said his farewell to the players on Friday.
Salford chief executive Ed Farish said: "We wanted to keep him. He's worked very hard here and he's got good football knowledge. But people make their own decisions. I think he's looking for new challenges."
Long's move comes just three days after the formal appointment of Brian Noble as the City Reds' new head coach and, although he has been linked with a move to either Wigan or St Helens, the timing is something of a surprise.
He had been offered the chance to stay on at the Salford City Stadium as assistant to Noble and, along with caretaker coach Alan Hunte, was preparing the team for Sunday's Tetley's Challenge Cup fourth-round tie against the University of Gloucestershire All Golds in Cheltenham.
"I am sorry to say I have handed in my notice at Salford City Reds," Long said on his Twitter account. "I am taking my time to weigh up my options."
Long is thought to have been offered the chance to return to his hometown club Wigan to coach their Under-19s, a post currently occupied by the Warriors' head of youth John Winder.