Jennings will replace Mike Redmond, who was fired along with bench coach Rob Leary just hours after Miami was two-hit by the Atlanta Braves in a 6-0 loss on Sunday. Jennings is the fourth Marlins manager in the past five seasons.
Jennings said that his own mother even asked him, "Are you crazy? Have you lost your mind?" when told of her son taking the job.
"There are going to be cynics. There are going to be critics. I fully expect to be judged on the body of work," Jennings said.
The team's vice president and assistant GM, Mike Berger, will take over Jennings' duties and Mike Goff will be the Marlins’ bench coach.
"Our expectation is this will go on as long as its going to go," Marlins president of baseball operations Michael Hill said. “We haven't played to our capabilities. We're hopeful that a new voice will spark and motivate our guys. I think we're just looking for some consistency."
MLB.com reported that special assistants Tony Perez, Andre Dawson or Jeff Conine were possible candidates for the job.
Redmond went 155-206 in two-plus seasons with Miami, including a 16-22 mark this season.
Jennings has never managed in the major leagues. He started out in the Marlins organization in 2002, working his way up from vice president of player personnel to assistant GM before being promoted to general manager in 2013.
While Miami is ninth in the National League in runs scored, pitching has been the biggest issue for the team.
Both the starter and bullpen ERA rank in the middle of the NL, and the team's eight blown saves are tied for the most in the major leagues.
Miami starts the week in fourth place in the National League East, six games behind the New York Mets after losing nine of its last 13 games.
- Scooby Axson