The German, who was dropped by Renault at the end of 2018, returned to the F1 grid last season when he replaced Sergio Perez and subsequently teammate Lance Stroll after they contracted the COVID virus.
Though he failed to start the British Grand Prix, he claimed points in his two other appearances for the team.
"I'll be working as a reserve driver for Aston Martin and also for Mercedes in selected races," he told ServusTV today.
The news comes at a time Mercedes has appointed former reserve, Esteban Gutierrez as a "brand and business ambassador".
"Esteban is a valued member of Mercedes," said Toto Wolff, "and his work behind the scenes since 2018 has made an important contribution to delivering maximum performance from the car at every race weekend.
"Since I have known Esteban, I have been impressed by his overall understanding of our industry and seen the potential for him to make a broader contribution to our success," he added. "He has supported us to develop valuable commercial relationships in the Latin American market, and he is well-placed to contribute more deeply to other areas of the team as well. Esteban's new position will enable him to add value across the team and we are excited to work with him even more closely."
However, it's understood that the Mexican, who previously raced for Sauber and Haas, no longer has the superlicence points to be eligible to participate in a Grand Prix should his services be required.
Check out our Friday gallery from Bahrain, here.