His last involvement in F1 was as an engineering consultant for Manor, which closed before the start of the 2017 season.
McLaren's struggles saw racing director Eric Boullier quit in July, while chief technical officer Tim Goss and chief engineer Matt Morris have also exited the team during the season.
“I think we're in a pretty good place now,” said McLaren CEO Zak Brown.
“We still need to kind of populate, ie James' (Key) start, but Andrea Stella is leading all performance, setting the targets, and it's for Pat as engineering director to execute, so it will ultimately be led by Andrea with Pat, the development of next year's car.
“And then of course there's lots of people that feed into that. Pete Prodromou from an aero, Mark Ingham and Neil Oatley from design, Simon Roberts the COO ultimately responsible for bringing everything together and Gil (de Ferran) on the sporting side but at the end of the day it's Andrea and Pat who are going to drive next year's car.”