Although mesopore designs are expected to play a key role in exploring electrocatalytic properties of carbons, facile preparation of mesoporous carbons (MPCs) with efficient dopants to enable high performance remains a great challenge. Herein, we for the first time introduce a self-growth-templating concept for the fabrication of three-dimensional (3D) N,P,Co-doped MPC frameworks, simply through pyrolysis of vitamin B₁₂ in NaCl assembly-enclosed nanoreactors. The route realizes the controllable formation of mesoporous templates and elimination of the templates, as well as in situ doping of N, P, and Co into MPCs in a one-step enclosed-space-assisted pyrolysis process. The highly mesoporous architecture not only enhances mass transportation but also provides the 3D electrocatalytic surface to expose highly active N,P,Co-induced dopants. These features endow MPCs with excellent activity as bifunctional electrocatalysts in oxygen reduction (E_1/2 of 0.85 V, J_K of 51 mA cm⁻² at −0.71 V and Tafel slope of 53 mV dec⁻¹) and CO₂ reduction reactions (overpotential of −0.19 V, maximum FE of 62% for CO and Tafel slope of 129 mV dec⁻¹), coupled with high electrochemical stability in aqueous electrolytes. We expect that the self-constructing vision can afford useful insights to guide the development of other advanced mesoporous materials other than carbon for broad applications.