Mr. Williams was long considered a leading candidate to replace Tim Cook as the company’s next chief executive.

Having worked at Apple for nearly three decades, Williams has played a significant role in introducing the iPod and iPhone programs, leading the development of the Apple Watch, and overseeing the design team,

Sabih Khan, Apple's senior vice president of operations, will assume the COO role later this month as part of what Apple called a long-planned succession. Williams will continue reporting to CEO Tim Cook in the interim while overseeing the Apple Watch,