Monitoring nicotinamide adenine dinucleotide (NADH) is important because NADH is involved in cellular redox reactions and cellular energy production. Currently, few biosensors quantify NADH in whole blood. However, they still have limitations due to several defects, including poor repeatability, long analysis time, and their requirement of extra sample pretreatment. In this study, we developed electrocatalytic sensors using screen-printed electrodes with a redox-active monolayer 4′-mercapto-N-phenylquinone diamine formed by a self-assembled monolayer of a 4-aminothiophenol (4-ATP). We exhibited their behavior as electrocatalysts toward the oxidation of NADH in whole blood. Finally, the electrocatalytic sensors maintained stability and exhibited 3.5 µM limit of detection, with 0.0076 ± 0.0006 µM/µA sensitivity in a mouse’s whole blood. As proof of concept, a polyhexamethylene guanidine phosphate–treated mouse model was used to induce inflammatory and fibrotic responses, and NADH level was measured for 45 days. This work demonstrates the potential of electrocatalytic sensors to analyze NADH in whole blood and to be developed for extensive applications.