In situ NMR investigations of a Kolbe electrolysis reaction using a 43 MHz ¹H NMR spectrometer were performed in this work. The electrochemical oxidative decarboxylation of biomass-derived valeric acid into the value-added product n-octane has been monitored. All reactions were conducted in non-deuterated methanolic solution using KOH as the supporting electrolyte. The working and counter electrodes consisted of Pt wire, and Ag wire was used as a pseudo-reference electrode. The influence of the magnetic field on the reaction kinetics as well as on mass transfer has been studied in detail. The findings show that the resulting mass transfer is highly dependent on the magnetic field. The significantly higher reaction velocity for in situ experiments is partly due to the strong Lorentz force, which agitates the solution and reduces the thickness of the electric double layer. The obtained results also suggest a strong influence of the magnetic field on the charge transfer from the electrode to the solution. The total resistance for the electrochemical reaction was significantly reduced by the presence of the magnetic field for all in situ experiments, at all points of the reaction. According to the reaction products, it was found that at high applied potentials (>5 V) or currents (>15 mA) the reaction velocity can be increased but evaporation and over-oxidation phenomena become more apparent. The results presented here show how NMR in situ electrochemistry can help to find optimal reaction conditions and improve quantitative analyses by example of a prominent green chemistry application.