Academy president Tom Sherak made the announcement Thursday.
This will be Ratner's first involvement with the show and Mischer's second.
Mischer produced and directed this year's telecast and was nominated for an Emmy. He is a veteran producer and director of live television events, including Super Bowl halftime shows, Olympics opening ceremonies and the Emmy Awards.
Ratner has produced and directed both film and television. His credits include "X-Men: The Last Stand," "Rush Hour" and the pilot of TV's "Prison Break."
The 84th annual Academy Awards will be presented at the Kodak Theatre on Feb. 26, 2012.