Two weeks after informing reporters "you're stuck with me, fortunately or unfortunately," Marrone made the surprise decision to step down Wednesday after the team's best finish in a decade.
In a statement released by the team, Bills owner Terry Pegula announced the second-year coach exercised the opt-out clause in his contract.
"We are disappointed that Coach Marrone will no longer be an important part of our organization," Pegula said. "We thank him for all of his hard work and leadership during his tenure, and wish him and his family the best with the next chapter in their lives."
Pegula said the team will immediately begin searching for a coach. One candidate might already be on the staff in defensive co-ordinator Jim Schwartz. He was hired a year ago after previously spending five seasons coaching the Detroit Lions.
The Bills have gone through six head coaches since Wade Phillips was fired following the 2000 season.
Marrone's decision to step down was a stunning one, coming after he led the Bills to a 9-7 finish. It was the Bills' first winning season since a 9-7 finish in 2004. Buffalo, however, missed the playoffs to extend the NFL's longest active post-season drought to 15 seasons.
"Surprised me. Didn't see it coming," centre Eric Wood said in a text message to The Associated Press.
Starting safety Aaron Williams blasted Marrone in a tweet.
"Lost all respect!!" Williams said.
The opt-out clause in Marrone's contract went into effect after Pegula and wife Kim bought the team in October. Marrone had until Wednesday to make his decision.
The team was sold after Hall of Fame owner Ralph Wilson died in March.
The timing of Marrone's departure comes as the Pegulas consider hiring a veteran presence to oversee the football operation. Former Bills GM turned broadcaster Bill Polian is considered a candidate to fill what would be a newly created position.
Marrone's status was still considered secure.
On Monday, Marrone sidestepped a question about his future when asked whether he would consider opting out of his contract.
"I do not discuss my contract. That's my rule. I never have and never will," he said.
Marrone is now free to explore other job openings.
Before stepping down, he had the backing of his players. That included veteran defensive tackle Kyle Williams who, on Monday, called it important for the team to maintain continuity after years of shuffling coaches and general managers.
"We've got an opportunity to be a good football team going forward," Williams said. "It would always be kind of a setback or something if you have a massive, massive overhaul."
Marrone went 15-17 in Buffalo. He was hired after four seasons at Syracuse, where Marrone helped revive his alma mater's struggling program.
A former NFL offensive lineman, Marrone was brought in because of his offensive expertise. That, however, never materialized with the Bills who were instead led by a dominating defence over the past two years.
The Bills offence finished 19th in yards gained in 2013, and regressed this year by finishing 26th. And Buffalo's running attack was particularly pedestrian this year in finishing with 1,482 yards rushing, the team's fewest in a 16-game season.
The offensive struggles were in part the result of a continuing revolving door at quarterback.
EJ Manuel, the team's first-round pick in the 2013 draft, proved injury prone in missing six games during his rookie season. This year, Manuel was benched following a 2-2 start.
He was replaced by 10-year journeyman Kyle Orton who finished the season as the starter. Orton, however, surprised the team Monday when he announced his intention to retire rather than return for another season.
Marrone had several eruptions this past season. He blew up at defensive end Jerry Hughes on the final day of training camp in August. Two weeks before the start of the season, Marrone had a heated exchange along the sideline with team president Russ Brandon.
Marrone, at times, was also terse with reporters in providing one-word answers.
He's not the first Bills coach to step down while still under contract.
Mike Mularkey quit the team for undisclosed reasons following the 2005 season after Tom Donahoe was fired as president and general manager.
Marrone was supposed to be part of a new era in Buffalo that began with Russ Brandon being elevated to president and CEO on Jan. 1, 2013.
Brandon hired Marrone. That spring, Doug Whaley was promoted to general manager after Buddy Nix stepped down.
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