Mechanical evaluation revealed significant variations between polymer composite specimens made by hand lay-up and resin transfer molding (RTM). This study aimed to evaluate the impact of manufacturing processes on important mechanical characteristics using E-glass/epoxy fiber-reinforced polymer matrix composites. When comparing RTM specimens to hand-lay-up specimens, a comparative analysis showed that the mechanical properties of the former had significantly improved. More precisely, RTM specimens demonstrated an amazing 28.51% improvement in tensile strength when compared to hand lay-up specimens. Likewise, RTM manufacturing resulted in a noteworthy 12.54% increase in flexural strength and a noteworthy 49.25% improvement in compressive strength over hand lay-up competitors. Additionally, impact strength tests showed that RTM specimens performed significantly better than hand lay-up specimens by 25.75%.