Rotherham have been unable to persuade manager Neil Warnock to extend his stay.
Ex-QPR manager Warnock, who was tempted out of retirement at the age of 67, joined the Millers in February until the end of the season.
After he had taken them from the relegation zone to Championship safety, the two parties discussed an extension of his deal.
However, they have been unable to reach an agreement and Warnock will now leave the club, along with first-team coaches Kevin Blackwell and Ronnie Jepson.
A club statement read: "Following lengthy and protracted discussions over the past two weeks, Rotherham United can confirm that Neil Warnock will not be the manager of the club going forward.
"We would like to place on record our thanks to Neil and his support staff for the 16 games that they led the team at the end of the 2015-16 campaign, and wish them health and happiness for the future.
"We will be looking to release details of a new managerial appointment in due course."
Warnock admitted he was all set to remain in the New York Stadium dugout before having a difficult rethink over the last 24 hours.
He is now set to find work at another Championship club and admits talks have already taken place.
"Up until a couple of days ago I thought I was staying," he said. "I've been in talks with two or three others clubs, and Tony [Stewart] knows about that.
"There will probably be more interest now the Rotherham decision is made.
"Leaving this club is one of the hardest decisions I've ever had to make. I love everything about it -- the chairman, the people, the fans, the family feel.
"I understand the chairman's need for a quick decision and the time is right for both parties to go their separate ways. There'll always be the memory of what we achieved together.
"Had I stayed, I think it would have been for the wrong reasons really. Up until a couple of days ago I thought I was staying.
"Rotherham is a project, and I want only one more year, They need someone to come in for two or three years, build things up bit by bit, consolidate their place in the Championship and then look to go on from there."
Warnock has written himself into Millers folklore with the job he performed in his 16-game stint, which saw him become the shortest serving manager in the club's history.
After three games of his reign, his side were six points adrift from safety and staring relegation in the face until he masterminded a 10-game unbeaten run, which saw them move 10 points clear of the drop zone and secure their place in the second tier.
Warnock juggled his time between the Millers and tending to his sick wife Sharon, who is battling cancer, and paid tribute to the fans who rallied around her.
They sent a large bunch of flowers to their Cornwall home, along with a collection of over £1,000 which was donated to charity.
"The love and affection those fans have shown Sharon is something else," Warnock said. "It's something I will never, ever forget."