De Bontin arrived at Monaco last April 2008 from Chicago and one of his main goals was to find new investors for the club.
"With its new investors, ASM FC has now the necessary means that could allow [the club] to regain its place among European and French soccer elite," Monaco said in a statement.
Monaco Sports Partenaires will replace Monaco Football Investissement as Monaco's majority owner, but a dispute between one of Monaco Sports Partenaires' major investors and de Bontin over his authority to run the club's affairs led to his decision to leave, sources told AFP.
De Bontin told Canal+ that he had considered his decision to step down for several months.
Monaco, the 2004 UEFA Champions League runner-up, had languished in the bottom half of Ligue 1. It is in a four-way tie for 12th place only three points above the relegation zone.
De Bontin, a college classmate and teammate of Prince Albert of Monaco at Amherst, met with a frosty reception from the French press, and Monaco fans criticized his moves, most notably the signing of American Freddy Adu on loan from Benfica. Adu has been a complete bust and has not even made the club's first 18 in recent weeks.