The former champions said they had decided to move away from a single technical director and create an executive team of three reporting directly to principal Andrea Stella.
Peter Prodromou takes on the role of technical director, aerodynamics while David Sanchez rejoins the team from Ferrari as technical director, car concept and performance.
Neil Houldey will fill the new role of technical director, engineering and design.
McLaren said Sanchez would not start work with the team until January 1st, 2024.
"It’s important now that we ensure we have a solid foundation as the next phase of our journey," said McLaren Racing chief executive Zak Brown in a statement.
"It has been clear to me for some time that our technical development has not moved at a quick enough pace to match our ambition of returning to the front of the grid.
"I’m pleased that, having completed a full review with Andrea, we are now able to implement the restructure required to set the wheels in motion to turn this around."
Key had been with McLaren since 2019, joining from Red Bull-owned Toro Rosso (now AlphaTauri).
McLaren finished last season fifth overall, after fourth in 2021, and have made a dismal start to 2023 with Australian rookie Oscar Piastri (21) and Britain's Lando Norris (23) last in the standings.
Their best finish so far is 15th, by Piastri in Saudi Arabia last Sunday.
Brown said before the start of the season in Bahrain this month that the team had missed some development targets with their new car.
Stella took over as principal in December, replacing Andreas Seidl who moved to Sauber.
"This new structure provides clarity and effectiveness within the team’s technical department and puts us in a strong position to maximise performance, including optimising the new infrastructure upgrades we have coming in 2023," said Stella of Thursday's changes.
