L-asparaginase (ASNase, EC 3.5.1.1) is an amidohydrolase enzyme known for its anti-cancer properties, with an ever-increasing commercial value. Immobilization has been studied to improve the enzyme’s efficiency, enabling its recovery and reuse, enhancing its stability and half-life time. In this work, the effect of pH, contact time and enzyme concentration during the ASNase physical adsorption onto pristine and functionalized multi-walled carbon nanotubes (MWCNTs and f-MWCNTs, respectively) with different size diameters was investigated by maximizing ASNase relative recovered activity (RRA) and immobilization yield (IY). Immobilized ASNase reusability and kinetic parameters were also evaluated. The ASNase immobilization onto f-MWCNTs offered higher loading capacities, enhanced reusability, and improved enzyme affinity to the substrate, attaining RRA and IY of 100 and 99%, respectively, at the best immobilization conditions (0.4 mg/mL of ASNase, pH 8, 30 min of contact time). In addition, MWCNTs diameter proved to play a critical role in determining the enzyme binding affinity, as evidenced by the best results attained with f-MWCNTs with diameters of 10–20 nm and 20–40 nm. This study provided essential information on the impact of MWCNTs diameter and their surface functionalization on ASNase efficiency, which may be helpful for the development of innovative biomedical devices or food pre-treatment solutions.