Polymeric nanocomposites and carbon-carbon composites are generally lighter and stronger and hold the key features for many industrial applications, such as aerospace, automotive, defense, electronics, biomaterials, sensors, energy, and consumer products. The objective of this study is to develop carbon-carbon and carbon-SiC nanocomposites by adding graphene into polyacrylonitrile (PAN) thermoplastic and carbon/SiC fibers where fiber reinforcement is used to enhance the mechanical properties leading to high strength-to-weight ratio nanocomposites. In this study, a mixing ratio of 20:80 for PAN and dimethylformamide (DMF) solvent was used to dissolve the PAN power using a magnetic stirrer. Graphene nanoflakes were then added to the solution at 0-4 wt.% ratio before making the nanocomposite coupons in aluminum grooves with carbon fibers/SiC fibers placed as reinforcement materials. After drying at room temperature, the coupons with PAN, graphene and carbon fibers/SiC fibers were oxidized at 200°C for 2 hours in air, and then carbonized in 650 °C for additional 2 hours in the presence of Argon (Ar) gas. Several material characterization techniques such as tensile tests, Fourier Transform Infrared Spectroscopy (FTIR), water contact angle, Scanning Electron Microscopy (SEM), Differential Scanning Calorimetry (DSC), flame retardancy (UL94), and electrical conductivity were conducted. It was found that the oxidized coupons of carbon fibers containing 4% graphene in PAN were found to be the superior ones in terms of mechanical strength, withstanding a maximum load of 9469.88 N and an Ultimate Tensile Strength (UTS) of 632.8 MPa at fracture. After conducting the water contact angle, all the coupons were found to be hydrophilic. It was found that the carbonized coupons were electrically conductive with a mean value of 344 S/m and fire retardant. While SEM helped to cross-check the right orientation of fibers, FTIR confirmed the presence of different functional groups of PAN and graphene. About 2% graphene carbonized and 4% graphene carbonized SiC reinforced coupons were found to have a Tg value of 126.13°C and 132.73°C, respectively.