The effects of graphene nanoplatelets (GNP) and multiwall carbon nanotube (MWCNT) hybrid nanofillers on the mechanical and thermal properties of reinforced polyethylene terephthalate (PET) have been investigated. The nanocomposites were melt blended using the counter rotating twin screw extruder followed by injection molding. Their morphology, mechanical and thermal properties were characterized. Combination of the two nanofillers in composites formulation supplemented each other which resulted in the overall improvement in adhesion between fillers and matrix. The mechanical properties and thermal stability of the hybrid nanocomposites (PET/GNP1.5/MWCNT1.5) were significantly improved compared to PET/GNP3 and PET/MWCNT3 single filer nanocomposites. However, it was observed that GNP was better in improving the mechanical properties but MWCNT resulted in higher thermal stability of Nanocomposite. The transmission electron microscopy (TEM) and field emission scanning electron microscopy (FESEM) revealed uniform dispersion of the hybrid fillers in PET/GNP1.5/MWCNT1.5 nanocomposites while agglomeration was observed at higher filler content. The MWCNT prevented the phenomenal stacking of the GNPs by forming a bridge between adjacent GNP planes resulting in higher dispersion of fillers. This complimentary geometrical structure is responsible for the significant improvement in the thermal stability and mechanical properties of the hybrid nanocomposites.