This report describes, for the first time, the coupling of UV–visible spectroscopy with multivariate curve resolution-alternative least-squares (MCR-ALS) algorithm to study peroxidase-like catalytic reaction of polyethylene glycol-functionalized poly (N-phenyl glycine) (PNPG-PEG) as an efficient and intrinsic peroxidase mimic activity (PMA) class of conducting organic polymer for selective detection of dopamine (DA) in the PNPG-PEG + TMB + H2O2 reaction system. PNPG-PEG was produced by means of a chemical route using ammonium persulphate (APS) as an oxidant agent of N-phenyl glycine monomer. The chemical composition, morphology, and thermal behavior of PNPG-PEG were examined by various instrumental techniques. PNPG-PEG exhibited significant peroxidase-mimic activity to catalyze the oxidation 3,3′,5,5′- tetramethylbenzidine (TMB) substrate to oxidized TMB (oxTMB). The qualitative and quantitative determination of the oxidized TMB can easily be detected by the naked-eye and the recorded UV–vis absorbance spectra at 652 nm, respectively. Owing to the superior peroxidase-mimic activity of PNPG-PEG, the colorimetric detection of dopamine was successfully achieved at pH 4.0. Under optimal conditions, acceptable linear dependency was recorded in the concentration range of 5.1–125.0 μM, with a limit of detection (LOD) and limit of quantification (LOQ) equal to 4.6 μM and 13.8 μM (S/N = 3), respectively. Furthermore, this colorimetric assay was successfully used for quantitative analysis of dopamine in fetal bovine serum (FBS) and horse serum (HS) samples with recoveries in the range of 97–105% and 100–122%, respectively. After resolving the bilinear data matrix using MCR-ALS, three chemical components were found for different concentrations and pure spectral profiles. Based on the resolved profiles, the presence of free, slightly penetrated, and majorly penetrated TMB molecules entering the polymeric structure can be easily detected using MCR-ALS as an available statical method without any complex separation instruments. This peroxidase mimetic nanozyme as a visual, simple, low-cost, sensitive, and reproducible colorimetric platform can provide great potential applications in the monitoring and diagnosis of dopamine-related diseases.