Reality shows such as "The Hills" and "A Shot at Love With Tila Tequila" has MTV on a ratings upswing. Norman also oversaw development of the popular video game "Rock Band," and guided the launch of separate networks for college students and young Latinos.
During a period in charge of VH1, she helped transform the network into a pop culture channel dominated by celebrity reality.
"Christina's mark on our company is indelible and it's worth noting that she reinvented herself and our businesses along the way," Toffler said.
Toffler will take over her duties until a replacement can be named.
