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,

Apple’s chief operating officer plans to hand the role off this month and retire later this year after roughly a decade as the tech giant’s No. 2 under Chief Executive Tim Cook.