Hybrid carbon reinforcements provide an effective route to enhance the mechanical and thermal performance of thermoplastics used in extrusion-based additive manufacturing. In this study, acrylonitrile-butadiene-styrene (ABS) nanocomposites reinforced with hybrid carbon nanotube (CNT) and graphene oxide (GO) reinforcements were fabricated using fused deposition modeling (FDM) and fused granular fabrication (FGF). A dissolution-assisted mixing strategy was employed to improve nanofiller dispersion prior to extrusion. Mechanical properties (tensile and flexural), thermal behavior (differential scanning calorimetry and thermogravimetric analysis), chemical structure (Fourier transform infrared spectroscopy), and microstructure (field emission scanning electron microscopy) of the specimens were comprehensively investigated. The results showed that the balanced 0.15%CNT-0.15%GO composition increased the tensile modulus by 23% in FDM and 25% in FGF, while tensile strength improved by 15% and 27%, respectively. In flexural testing, the 0.1%CNT-0.2%GO composition fabricated via FGF exhibited the best performance, with 114% and 63% improvements in flexural modulus and flexural strength, respectively. Thermal analysis confirmed increases in glass transition temperature by up to 5.4°C and 5% weight loss temperature by up to 15°C in hybrid nanocomposites. Microstructural observations revealed uniform nanoparticle dispersion, reduced porosity, and crack deflection and bridging mechanisms in hybrid specimens. Although FDM provided higher absolute mechanical properties, FGF demonstrated more significant relative improvements, particularly in flexural properties. The findings indicate that the selection of fabrication method and optimal reinforcement ratio plays a decisive role in the reinforcement efficiency of hybrid reinforcements in additive manufacturing.