Schumacher took the wheel of McLaren's 2021 MCL35M car on Wednesday as part of F1's so-called TPC rules, which allow tests with two-year-old machinery.
The test is part of Mercedes' arrangement of sharing its test and reserve drivers with McLaren, which takes customer engines from the German manufacturer.
In previous years it had agreed a deal with Mercedes to call upon Stoffel Vandoorne and Nyck de Vries in case it needed to replace one of its race drivers.
For the 2023 season that deal was extended, with incoming reserve driver Schumacher being made available if either Lando Norris or rookie Oscar Piastri is unable to race.
By completing the Portimao test Schumacher was able to get up to speed with how McLaren works, having not operated with the Woking team before.
With McLaren's race team in the middle of a double-header of the Austrian and British grands prix, the team sent out a test team which included some of its young engineering talents.
"Joining Mick are some of our mechanic and engineering apprentices and interns, gaining valuable experience working in a trackside environment," the team explained on Twitter.
Source: Autosport