Carbon dioxide (CO2) has been widely known to be the main contributor to global warming and temperature fluctuations. The CO2 gas is primarily found in the combustion of fossil fuels and natural gases. Several approaches were considered in the industry to minimise CO2 emissions such as ammonia scrubbing and membrane technology. In recent years, membrane technology has exhibited excellent CO2 separation performance especially the mixed matrix membrane (MMM) due to its combined properties of organic and inorganic materials. The small footprint and high efficiency of the membrane technology compared with the traditional gas separation processes has given it a distinct advantage. In this work, the MMMs are synthesised from the cellulose acetate butyrate (CAB) polymer and the functionalised multi-walled carbon nanotubes (MWCNTs) through Chen’s soft-cutting method. This study aims to synthesise an excellent MMM with high separation performance by studying the effects of incorporating different amounts of MWCNTs fillers into the CAB blended membrane. The fabricated MMM was developed using 4 wt% of CAB with molecular weights of 12000, 65000 and 70000 mixed in a ratio of 1:1:1. The functionalised MWCNTs were incorporated into the CAB matrix with different filler loadings ranging from 0.0125 wt% to 0.2 wt%. The separation performance of the fabricated MMM was successfully conducted towards CO2/N2 separation. The highest CO2 and N2 gas permeance were exhibited from the MMM-0.025 with average values of 36.0 ± 0.4 GPU and 28.0 ± 0.2 GPU, respectively. Further, the highest CO2/N2 selectivity exhibited from the MMM-0.025 was 1.3 ± 0.1. The outcome of this research confirmed the positive effects of different Mn and MWCNTs filler amounts on the performance of the MMM.