Brian Lesser, who oversaw billions in ad spending at GroupM, is leaving for a new job at AT&T

Brian Lesser, North American CEO at the ad buying giant GroupM, is leaving for a new job at AT&T.

Lesser, who oversaw billions in ad spending at the WPP-owned media agency, was named CEO in 2015. GroupM oversees more than $100 billion in marketers' ad budgets.

Prior to that, he was instrumental in the agency's embrace of 'programmatic' ads, helping build and run the GroupM specialty buying unit Xaxis.

At AT&T, Lesser will serve as CEO of new business centered on advertising, content, data and analytics. He'll report directly to AT&T Chairman and CEO Randall Stephenson.

Lesser joins AT&T at an intriguing time. The company is seeking approval of a mega deal to acquire Time Warner - one that could have massive implications for the media and advertising industries.

To date, AT&T has been something of a sleeping giant in advertising. The company at one point had built and in house mobile ad network, which it eventually shuttered. More recently it has focused on using the company's DirecTV asset to help advertisers target specific households with ads. With Lesser's hiring, it appears the telecom firm may be looking to take its ad game up a notch.