Dispersion-corrected periodic DFT calculations have been applied to elucidate the Langmuir–Hinshelwood (dissociative) and an Eley–Rideal/Mars–van Krevelen (associative) mechanism for ammonia synthesis over Co3Mo3N surfaces, in the presence of surface defects. Comparison of the two distinct mechanisms clearly suggests that apart from the conventional dissociative mechanism, there is another mechanism that proceeds via hydrazine and diazane intermediates that are formed by Eley–Rideal type chemistry, where hydrogen reacts directly with surface activated nitrogen, in order to form ammonia at considerably milder conditions. This result clearly suggests that via surface defects ammonia synthesis activity can be enhanced at milder conditions on one of the most active catalysts for ammonia synthesis.