A simple colorimetric method for the quantitative detection of two cephalosporins, cefoperazone (CFZ) and cefepime (CPM), has been developed and validated for the application in clinical and quality control laboratories. The method was based on exploiting the reducing properties of cephalosporins due to the 7-aminocephalosporanic acid dihydrothiazine and a β-lactam ring to produce gold (GNPs) or silver (AgNPs) nanoparticles from their corresponding metal solutions. The resulting nanoparticles exhibited surface plasmon resonance signals at 530–550 nm or 430 nm for GNPs or AgNPs, respectively, with an intensity dependent on drug concentration that permits the quantitative analysis. Furthermore, the resulting nanoparticles were characterized using a dynamic light scattering instrument and transmission electron microscopy. Factors influence the reaction, such as the reagents volume, pH, temperature and reaction time, have been studied and optimised. In addition, the study was validated according to the official guidelines and extended to evaluate the antimicrobial activity to assess potential drug degradation during the analysis. Overall, the method developed in this study can be used for the quantitative analysis of CFZ and CPM in pure forms and pharmaceutical preparations with relatively good sensitivity and selectivity as well as, acceptable accuracy and precision.