Philip Anschutz will step into a more active role with the company, which is owner of the LA Galaxy, among many other properties.
Additionally, the company, which had been on the market, is no longer for sale.
Dan Beckerman will assume the position of President and CEO, while company chairman Philip Anschutz will resume a more active role in the company.
Leiweke had been in his position since 1996, and was integral in bringing David Beckham to MLS as the league's first Designated Player.
"We appreciate the role Tim has played in the development of AEG, and thank him for the many contributions he has made to the Company. We wish him well in his new endeavors," Anschutz said in a statement.
AEG owns much more than just sports franchises, as it has more than 120 arenas, stadiums, and theaters on five continents. It also contains AEG Live, the world's second largest concert promotion and touring company. The company was put up for sale in September 2012.
"From the very beginning of the sales process, we have made it clear to our employees and partners throughout the world that unless the right buyer came forward with a transaction on acceptable terms we would not sell the Company," Anschutz said.