This paper discusses the electrochemical polymerization of basic dye films, which are composed of basic red 9 (BR9), on various electrodes and the enhancement of the electropolymerization by functionalized multiwall carbon nanotubes (f-MWCNTs) modification of the electrode surface. The presence of f-MWCNTs enhances the surface coverage (Γ) and stability. Poly(BR9) films were electrocatalytically active for epinephrine and serotonin oxidation. The electrocatalytic oxidation current developed from the anodic peak of the redox couple. Electrochemical impedance spectroscopy (EIS) was applied to monitor the whole process of the electrode modification. EIS can provide useful information regarding the impedance changes on the electrode surface between each step. We studied the surface morphology of the composite film using scanning electron microscopy (SEM) and atomic force microscopy (AFM), which revealed that BR9 is doped on f-MWCNTs. Cyclic voltammetry (CV) was used for the measurement of the electroanalytical properties of the analytes. The sensitivity values for the f-MWCNTs/BR9 composite film were higher than the poly(BR9) and f-MWCNTs composite film. Finally, differential pulse voltammetry (DPV) was used for the detection of a mixture of analytes at the f-MWCNTs/BR9 composite film. We simulated a more complex system with both serotonin and epinephrine present simultaneously. This system also exhibited oxidation peaks for serotonin in bovine calf serum (BCS) and epinephrine injection for real samples determination at pH 7.0 at the f-MWCNTs/BR9 composite film.