The Haber-Bosch process for the synthesis of ammonia (NH3) not only causes large energy consumption but also leads to CO2 emissions. Electrocatalytic hydrogenation of N2 to NH3 under ambient conditions is a highly desirable alternative method; however, it needs an efficient electrocatalyst. In this study, we report a mesoporous boron nitride (MBN) metal-free electrocatalyst for the N2 reduction reaction under ambient conditions. Owing to its mesoporous structure, this MBN electrocatalyst can expose more active sites, resulting in an outstanding NH3 formation rate of 18.2 μg h-1 mgcat.-1 with a faradaic efficiency of 5.5% at -0.7 V vs. reversible hydrogen electrode in 0.1 M Na2SO4. It also demonstrates strong long-term electrochemical durability.