MXenes and doped carbon nanotubes (CNTs) have entered into research arenas for high-rate energy storage and conversion. Herein, a method of postsynthesis of MXenes in boron, nitrogen codoped CNTs (BNCNTs) is reported with their electrocatalytical hydrogen evolution performance. The encapsulation of hexagonal molybdenum nitrate nanoparticles (h-MoN NPs) into BNCNTs protects h-MoN NPs from agglomeration and poisoning in the complex environment. In principle, the synergism of B and N dopants on the doped CNTs and confined h-MoN NPs produces extremely active sites for electrochemical hydrogen evolution. Density functional theory calculations reveal that the active sites for hydrogen evolution originate from the synergistic effect of h-MoN(001)/CN (graphitic N doping) and h-MoN(001)/BNC. The h-MoN@BNCNT electrocatalyst exhibits a small overpotential of 78 mV at 10 mA cm⁻² and Tafel slope of 46 mV per decade, which are dramatically improved over all reported MoN-based materials and doped CNTs. Additionally, it also exhibits outstanding electrochemical stability in environments with various pH values and seawater media from South China Sea.