High-performance mixed-matrix membranes (MMMs) were developed to help control the global warming by capturing and sequestrating carbon dioxide (CO2) from post combustion flue gas originating from burning of fossil fuels. MMMs of different compositions were prepared by synergistically doping glassy polymer Ultrason((R)) S 6010 (US) with nanosheets of graphene oxide (GO) and nanocrystals of zeolitic imidazolate frameworks (ZIF-300) in varying degrees. Solution-casting technique was used to fabricate various MMMs to optimize their CO2 capturing performance from a CO2-enriched gas mixture. The prepared composite membranes indicated enhanced filler-polymer interfacial adhesion, consistent distribution of nanofillers, and thermally stable matrix configuration. Owing to the synergistic effect of incorporated nanofillers, CO2 permeability, and CO2/N-2 permselectivity of the membranes were enhanced as demonstrated by gas sorption and permeation experiments. As compared to neat Ultrason((R)) membrane, CO2 permeability and CO2/N-2 ideal selectivity of fabricated MMMs doped with 30wt% ZIF-300 nanocrystals and 1wt% GO nanosheets were considerably improved. Key features of the fabricated MMMs include their structural and thermal stability coupled with improved gas separation performance.