I thought Mearls has been there for a while. I can't imagine he wasn't at least in the mix during the design phase of Essentials.
But, was he the leader of this revolution or simply a contributor?
Or, was this stuff in the pipeline before he took over.
A quick search indicates that the Essentials was in the pipeline long before Mike Mearls became the manager of D&D.
15 May: Announcement of Mike's new position
26 March: ENWorld news article mentioning the Essentials
(And I'm pretty sure there were even earlier mentions of the Essentials line.)