Apple has announced an executive shakeup today that sees Jeff Williams promoted to Chief Operating Officer and marketing chief Phil Schiller taking over App Store leadership across all Apple platforms.

Williams, who joined Apple in 1998 as head of worldwide procurement, becomes Apple's fourth C-level executive alongside CEO Tim Cook, Chief Financial Officer Luca Maestri and Chief Design Officer Jony Ive.

As vice president of operations since 2004, his responsibilities included overseeing Apple's supply chain, service and support, and the company's social responsibility initiatives protecting its employees worldwide. He also continues to supervise development of the Apple Watch and ResearchKit.

Cook called Williams "hands-down the best operations executive I’ve ever worked with."

“We are fortunate to have incredible depth and breadth of talent across Apple’s executive team. As we come to the end of the year, we’re recognizing the contributions already being made by two key executives,” said Tim Cook, Apple’s CEO. “Jeff is hands-down the best operations executive I’ve ever worked with, and Johny’s team delivers world-class silicon designs which enable new innovations in our products year after year.”

Apple's COO position had been vacant since Cook was named CEO in August 2011.

Schiller's expanded role will be focused on strategies to extend Apple's ecosystem across iPhone, iPad, Mac, Apple Watch and Apple TV, alongside his traditional marketing responsibilities. App Store leadership previously belonged to Apple services chief Eddy Cue, who continues to oversee the iTunes Store, Apple Music, Apple Pay, Apple Maps, iAd, iCloud, iWork apps and more.

Cook continued, “In addition, Phil is taking on new responsibilities for advancing our ecosystem, led by the App Store, which has grown from a single, groundbreaking iOS store into four powerful platforms and an increasingly important part of our business. And I’m incredibly happy to welcome Tor Myhren, who will bring his creative talents to our advertising and marcom functions.”

Apple also announced that Johny Srouji is joining the company's executive team as Senior Vice President for Hardware Technologies, and that Grey Group chief creative officer Tor Myhren will join Apple in the first calendar quarter of 2016 as Vice President of Marketing Communications, reporting to CEO Tim Cook.

Srouji joined Apple in 2008 to lead development of the A4 chip, and now oversees the company's silicon and hardware technologies, including batteries, application processors, storage controllers, sensors silicon, display silicon and other chipsets across Apple's entire product line. He previously worked at Intel and IBM in the areas of processor development and design.

Myhren will be responsible for Apple's advertising and marketing communication efforts across a range of creative disciplines, including video, motion graphics, interactive web design, packaging and retail store displays. The experienced advertising executive will succeed Hiroki Asai, who is retiring after 18 years in graphic design and marketing communications roles at Apple.

Update: While Williams is often described as "Tim Cook's Tim Cook," The Wall Street Journal has learned that his new COO title is more about formalizing a role he had within Apple, as opposed to grooming him to be the next Apple CEO.

A senior executive at the company, who declined to be identified, said the move isn't necessarily a sign that Mr. Williams is the heir apparent to Mr. Cook and that many members of the current executive team have what it takes to be the future CEO. The new title formalizes a role that Mr. Williams has been playing inside the company, the executive said.

The leadership changes are reflected on Apple's executive profiles page.