The American Hockey Club introduced Keefe at a news conference Monday. Keefe joins the Marlies after spending the last three seasons behind the Greyhounds bench, helping turn the OHL club into a contender.
"It's an incredible opportunity to grow as a coach," Keefe said.
He joins former Greyhounds general manager Kyle Dubas in the Maple Leafs organization. Dubas was named the Leafs assistant GM last year.
"You want to have the coach that can help us be the best developer of our players," Dubas said.
The AHL's Marlies are the farm team for the Leafs. Keefe will be tasked with helping Toronto's prospects develop into NHL regulars.
"They want to be NHL players. We want them to be NHL players," Keefe said. "We're going to attack each day with that in mind."
Keefe earned both the 2014-15 CHL and OHL coach of the year awards after leading the Greyhounds to a league-best record of 54-12-0-2 and establishing a new franchise record with 110 points. The team's .809 winning percentage and 342 goals scored led the entire Canadian Hockey League this past season.