To investigate the influence of varying concentrations and types of carbon fiber (CF) on the mechanical properties of polyamide 6 (PA6) composites, this study explores the mechanical properties of PA6 composites with various CF types and quantities. The micro-morphology of the composites and the CF length distribution were characterized. The results indicate that the inclusion of carbon fibers significantly enhances the tensile, flexural, and notched impact strengths of PA6. Specifically, when about 30 wt% of CF T300 was added, the tensile and flexural strength of the composite reached a maximum of 166 MPa and 224 MPa, respectively, representing increases of 236.6% and 229.6%, respectively, compared to pure PA6. The maximum flexural modulus achieved 14.6 GPa, which was six times as large as that of pure PA6. Moreover, the CF length in the PA6 matrix follows a near-Gaussian distribution. A proper CF length and orientation, along with strong interfacial bonding between CF and the PA6 matrix, contribute to improved mechanical properties. The overall performance of T700-reinforced composites is better than that of T300-reinforced ones due to T700’s higher precursor strength and better fiber-length retention. This study provides guidance for fabricating high-performance PA6 composites.