Time Warner abruptly fired the two top leaders of its struggling AOL unit Wednesday and replaced them with one of the top executives from Google.

AOL’s new chairman and chief executive will be Tim Armstrong, who joined Google in 2000 to start a sales operation for the then-tiny search engine. Since then, Google has grown to sell more advertising than any newspaper, magazine or television network. Mr. Armstrong, who was the president of American operations, looked after both advertising sales and relationships with publishers.

He replaces Randy Falco, a former top executive of NBC, who was brought in to run AOL in November 2006 by Jeffrey L. Bewkes, Time Warner’s chief executive.

Ron Grant, a former aide to Mr. Bewkes who was the No. 2 executive at AOL under Mr. Falco, was also fired Wednesday.