SARS-CoV-2 is the causative agent of COVID-19. Mpro is the main viral protease, with a critical role in replication and, therefore, an attractive target for antiviral drug discovery. The clinically approved drug nirmatrelvir from Pfizer, and the clinical candidate ensitrelvir from Shionogi Pharmaceuticals had so far showed great potential for treatment of viral infections. Despite the importance of new therapeutics, the broad use of antivirals is often associated with mutation selection and resistance generation. Herein, we characterized 14 naturally occurring polymorphisms that are already in circulation and are within the radius of action of these two antivirals. Nirmatrelvir retained most of its in vitro activity against most polymorphism tested, while mutants G143S and Q189K were associated with higher resistance. For ensitrelvir, higher resistance was observed for polymorphisms M49I, G143S and R188S, but not for Q189K, suggesting a distinct resistance profile difference between the two inhibitors. The crystal structures of selected polymorphism reveal the structural basis for resistance generation. Our data will assist the monitoring of potential resistant strains, support the design of combined therapy to avoid resistance, as well as assist the development of a next generation of Mpro inhibitors.