Multi-walled carbon nanotubes (MWCNTs) were immobilised with cobalt nanoparticles and analyzed by transmission electron microscopy. This modification procedure substantially improved colloidal dispersion of the immobilised MWCNTs in water and organic solvents, yielding uniform and stable thin films for modification of the glassy carbon electrode surface. The modified electrode showed an efficient catalytic role for the electrochemical oxidation of thioridazine (TR), leading to remarkable decrease in its oxidation overpotential of approximately 100 mV and enhancement of the kinetics of the electrode reaction, which can be confirmed by increasing in the peak current and sharpness of the peak. A remarkable enhancement in microscopic area of the electrode together with the catalytic role of the composite modifier resulted in a considerable increase of the peak current (∼55 times), and negative shift in the oxidation potential of TR. The effect of the thickness of the modifier on the GCE surface was optimized by monitoring its cyclic voltammetric responses toward TR. The mechanism of the electrocatalytic process on the surface of the modified electrode was analyzed by obtaining the cyclic voltammograms at various potential sweep rates and pHs of the buffer solutions. Differential pulse voltammetry was applied as a very sensitive analytical method for the determination of sub-micromolar amounts of TR. A linear dynamic range of 5.0 × 10−7 to 1.0 × 10−4 M with a detection limit of 5.0 × 10−8 M TR was obtained. The prepared modified electrode shows several advantages such as simple preparation method, high stability and uniformity in the composite film, high sensitivity, and excellent catalytic activity in physiological conditions, long-term stability and remarkable voltammetric reproducibility. These excellent properties make the prepared sensor suitable for analysis in pharmaceutical and clinical preparations. The modified electrode was successfully applied for the accurate determination of minor amounts of TR in pharmaceutical and clinical preparations.