Kirk McDonald. a top executive at WarnerMedia’s Xandr ad-tech unit, was named the new CEO of North American operations of WPP’s large GroupM media-buying firm, marking the latest in a string of top advertising leaders to leave the AT&T media operation.

GroupM is one of the largest and most influential media-buying operations in the industry, managing about $17.6 billion in media billings across the U.S. and Canada. McDonald will take over the role on September 15. The Wall Street Journal previously reported on McDonald’s hire.

“We were looking for someone who had both the right experience in media and the capability to lead the region through transformational change. Not only did Kirk demonstrate he has both, but his vision for making advertising work better for people aligned perfectly with ours,” said Christian Juhl, GroupM’s global CEO, in a prepared statement. “His outstanding reputation in the industry put him at the top of our list. Additionally, he has worked in complex organizations, so he understands the dynamics at play in a media holding group.”

McDonald had been chief business officer at Xandr, a unit launched by AT&T in preparation for its $85.4 billion acquisition of the entertainment company formerly known as Time Warner. Xandr had operated as a stand-alone unit with a reporting line directly into former AT&T CEO Randall Stephenson. Xandr was built to develop new kinds of ad-placement technologies that could harness insights from AT&T’s DirecTV subscribers and also use data to help place commercials more precisely amid a sea of different media environments,. But its original CEO, Brian Lesser, departed abruptly, and Xandr was eventually made a part of WarnerMedia’s overall ad-sales efforts.

McDonald is the latest in a series of senior ad-sales executives to leave AT&T and WarnerMedia. In addition to Lesser, the companies have also parted ways with David Levy, the former Turner president who had oversight of most revenue streams, and Donna Speciale, the former president of WarnerMedia ad sales, who has yet to be repalced. WarnerMedia is said to be close to naming a new executive for that role.

Mike Welch, a longtime AT&T executive was named head of Xandr. Welch was previously executive vice president of product and business development at Xandr after being promoted from senior vice president, corporate strategy and business development in September 2019.

GroupM has also suffered from a bout of executive departures. McDonald replaces Juhl in the role, who replaced former North American CEO Tim Castree. He departed the company in 2018.

Before joining AT&T, McDonald was president of PubMatic, a marketing-automation software company, and held roles with Time Inc., Fortune|Money Group and CNET Networks, among others.