Which Argentine racing driver won F1 championships with Mercedes in 1954 and 1955?

Which Argentine racing driver won F1 championships with Mercedes in 1954 and 1955? Juan Manuel Fangio did.

Fangio was already a world champion before joining Mercedes. He won the 1951 title with Alfa Romeo, then captured his second championship in 1954 after beginning the season with Maserati and moving to Mercedes when the team introduced its new W196.

Mercedes made its 1954 championship debut at the French Grand Prix, where Fangio won and Karl Kling finished second. Fangio then added victories in Germany and Switzerland, securing the title before the season ended. The W196 combined a sophisticated straight-eight engine with advanced engineering for its era.

In 1955, Fangio stayed with Mercedes and won his third World Drivers’ Championship, taking victories in Argentina, Belgium, the Netherlands and Italy. His teammate Stirling Moss finished second in the standings, which is the most common mix-up in this question: Moss was a celebrated Mercedes driver, but not the champion in either of these seasons.