One of his biggest challenges will be balancing the company's dependence on China while parrying acute political pressure from the White House.
The handover ends Tim Cook's 15-year run, replacing the operations specialist who built Apple's supply chain with the engineer who built its hardware.
Ternus, 51, joined Apple's design team in 2001 and worked his way up to senior vice-president of hardware engineering, reporting to Cook.
He leads the engineering teams behind Apple's entire product line-up, including the iPhones that generate most of the company's revenue.
He gets barely a week to settle in, with Apple holding its annual iPhone event on September 9, where it is expected to unveil its first foldable handset.
