Syngas production from catalytic gasification of polystyrene and pinewood in CO2 atmosphere was investigated over Ni-Mg/Al2O3 catalyst in a fixed-bed reactor at 900 °C. A quasi in situ method was adopted for catalytic gasification wherein the catalyst placed downstream of the feedstock in the same reactor was used for enhanced syngas production. The effect of catalyst on evolutionary behavior, cumulative syngas yield, syngas composition, and cold gas efficiency was systematically analyzed. The results showed that addition of catalyst for polystyrene gasification resulted in enhanced yields of 63% H2, 20% CO, 119% CH4, and 85% C2-C3 yields. Enhanced H2 and light hydrocarbon yields were mainly from enhanced cracking of pyrolytic vapors from polystyrene degradation, while the CO yield was attributed to CO2-assisted reforming of benzene derivatives from primary cracking and polycyclic aromatic hydrocarbons (PAHs) from secondary gas phase condensations. The yields of H2, CO, CH4, and C2-C3 from pinewood gasification in the presence of catalyst was also enhanced by 150%, 14%, 39%, and 16%, respectively, indicating that Ni-Mg/Al2O3 catalyst can efficiently enhance syngas production in CO2-assisted gasification. A comparison of syngas composition between non-catalytic and catalytic conditions revealed improved syngas quality in catalytic gasification with increased H2 mole fraction but decreased CO mole fraction. Furthermore, cold gas efficiency enhanced from 44% to 57% in catalytic polystyrene gasification, and from 75% to 94% in catalytic pinewood gasification. The results suggest that catalytic CO2 gasification offers a promising pathway for efficient energy production from wastes plastics and biomass while simultaneously using CO2.