Ron Washington cited personal reasons as he surprisingly resigned as Texas Rangers manager on Friday.
The 62-year-old took the role in 2007 and guided the MLB outfit to their only two World Series appearances in 2010 and 2011.
With a 53-87 win-loss record, the Rangers have the worst figures in the league this season and are the only team eliminated from play-off contention.
In a statement, Washington revealed plans were already being put in place for 2015, but off-field matters led to his resignation.
"Today, I have submitted my resignation from the job I love - managing the Rangers - in order to devote my full attention to addressing an off-the-field personal matter," he said.
"As painful as it is, stepping away from the game is what's best for me and my family.
"This is in no way related to the disappointing performance of the team this season. We were already discussing 2015 and looking forward to getting the Rangers back to post-season contention.
"I deeply regret that I've let down the Rangers organisation and our great fans.
"Over the past eight seasons, it's been a privilege to be part of some of the best years in club history and I will always be grateful for the opportunities I've had here, and for the great management, players, and coaches who have made our time here a success.
"Thank you for respecting my privacy."
Tim Bogar will be interim manager for the remainder of the season.