The major limitation of the widely used Additive manufacturing technology, Fused Filament Fabrication is the low mechanical strength of printed geometries. Investigations into the impact of process variables and thermal annealing on the tensile strength of composite carbon fiber polylactic acid thermoplastics to overcome the above-mentioned issue were the main aspects of this study. ASTM D638 Type IV tensile models of 20% carbon-infused polylactic acid were used for the investigations. The selected process parameters were infill density, infill pattern, nozzle temperature, layer height, and print speed. Results reveal that the specimen with 90% infill density, gyroid pattern of printing at 230 °C nozzle temperature with a layer height of 0.1 mm and print speed of 40 mm/s exhibited the maximum tensile strength of 37.27 MPa. Further, thermal annealing was carried out at 65 °C, 95 °C, 125 °C, and 155 °C considering the glass-transition temperature and melting point for a duration of 30, 60, 120, and 240 min as a post-treatment technique to improve the mechanical characteristics of the specimen. Annealing done at 95 °C for 120 min enhanced the tensile strength by 14%, thus giving an overall strength of 42.49 MPa. Hence, the optimum selection of process parameters and post-processing conditions could significantly improve the mechanical strength of fused filament fabricated thermoplastics.