This study examines the mechanical properties of two widely used thermoplastic polymers, polyethylene terephthalate glycol (PETG) and polylactic acid (PLA), in the context of 3D printing using fused deposition modeling (FDM). It focuses on how they react to different infill densities, including 20, 40, 60, 80, and 100%. This study’s objective is to thoroughly examine the effects of various infill densities on these materials’ mechanical characteristics and offer practical advice for improving the functionality of 3D-printed parts. PETG and PLA specimens are 3D printed for this experiment using an FDM printer with a layer height of 0.2 mm and a raster angle of 45°. The tensile properties of the printed specimens are then thoroughly evaluated using a universal testing machine (UTM). To identify the main reasons for failure and clarify the impact of infill density on these results, microstructural analyses of fractured specimens are also carried out using a microscope with a scanning electron microscope (SEM). This extensive research not only improves our understanding of how PETG and PLA behave mechanically at various infill densities but also provides designers, engineers, and manufacturers with valuable suggestions for customizing infill density to enhance the mechanical performance of 3D-printed parts. The findings of this study show that with the increase in infill density, both the materials are showing improvement in the tensile strength and 100% infill density is showing better results as compared to other infill densities for PETG as well as PLA.