The present study provides an efficient process for the high-yield production of formic acid (24%) by reduction of carbon dioxide (CO2) with potassium borohydride at ambient conditions. The effects of reaction temperature, CO2 pressure and borohydride concentration have been investigated. For a 0.5M borohydride solution, 0.15mol/L of formic acid were produced at room temperature and ambient pressure with yields increasing at higher pressures. A time-resolved in situ 1H and 11B nuclear magnetic resonance (NMR) technique was firstly developed to monitor the elementary reaction processes under real working conditions. Direct evidence is given for the formation of H2, HD and a hydroxyborohydride intermediate (BH3OH-) formed during borohydride decomposition indicating that the source of the hydrogen gas comes from both the borohydride anion and water, while borohydride works as a water-splitting reagent. Consequently, a reaction mechanism involved in both borohydride hydrolysis and CO2 reduction has been established.