Norris and Sainz have played key roles in McLaren's long-promised revival in 2019.
McLaren are fourth in the constructors' standings, with that 'best of the rest' status backed up by Sainz and Norris sitting seventh and eighth respectively in the drivers' championship.
A statement read: "The pair have integrated well into the team over the past nine months as McLaren progresses on its road to recovery.
"Retaining both drivers for 2020 echoes the team’s commitment to consistency and continuity, which are key pillars of the team’s return to competitiveness."
The news comes soon after reports that McLaren and Fernando Alonso had "parted ways" with no racing plans for the future. Alonso has been often linked with an F1 return in 2020, but it will not be happening with his former employers, where he remains an ambassador.
McLaren team principal Andreas Seidl added: "The performance of both Carlos and Lando have been a key part of the team’s progress.
"Their feedback has enabled our engineers to develop the MCL34 and look ahead to the MCL35.
"Carlos has played a key role in developing the car using his experience, while Lando has got up to speed with the demands of Formula 1 incredibly quickly and is growing in confidence every weekend.
"This is welcome news for the whole McLaren team and we look forward to continuing our partnership together."
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