For supported catalytic systems, size and dispersion of active sites is a predominant feature that determines the performance. While molybdenum carbide (Mo2C) is deemed as potential electrocatalyst for the hydrogen evolution reaction (HER), growth of Mo2C with small particle size and uniform dispersion on carbon support remains a challenging endeavor, predominantly due to the requirement of high crystallization temperature (>= 750 degrees C). Herein, a molecular approach is demonstrated to modify the self-assembling of molybdenum on carbon nanotubes, steering a discrete nucleation and growth of Mo2C. Molybdenum reacts with oxalate forming molybdenum-oxalate, which presumably improves immobilization (through interaction between carbon surface and oxygen of oxalate) and increases the inter-molybdenum-distance (through steric hindrance). This oxalate assisted assembling of molybdenum inhibits the agglomeration and coalescence of Mo2C during growth. The as-prepared electrode exhibits a remarkable HER and stability with low onset and overpotential, and small Tafel slope. The high activity is rationalized in terms of size, dispersion, specific and electrochemically active surface area, electrical conductivity, interfacial charge transfer resistance, and turnover frequency. Effect of growth temperature and catalyst's mass on the performance is investigated, and how electrochemically active surface area, interfacial charge transfer resistance and performance vary with respect to molybdenum carbide contents is established. (C) 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.