The production of ammonia (NH3), an important carbon-free chemical, through nitrogen (N2) fixation under mild conditions, is one of the most challenging and attractive chemical processes for industrial applications. However, most N2 fixation occurs through transition-metal based systems and examples of metal-free catalysts remain elusive. Herein, by means of first-principles computations, we demonstrate that dynamical as well as highly thermally stable (up to 800 K) single boron atom doped nanoporous carbon nitride materials, i.e. C2N monolayers, are a potential metal-free single atom catalyst for efficient N2 fixation under visible light absorption. Based on the B-N synergistic effect, N2 strongly binds to the B/C2N surface through end-on and side-on modes respectively. Our computation reveals that the single B atom doped C2N-concept catalyst could effectively reduce N2 to NH3 with a record low onset potential (0.18 eV) through enzymatic pathways and can sufficiently suppress the competing hydrogen evolution reactions. Multimodal binding of gas phase N2 molecules with selective stabilization of NxHx by proton-electron (H+ + e-) pairs leads to the highest catalytic performance of B/C2N. Moreover, deposition of single B atoms on C2N dramatically enhances the absorption of light in the visible and IR regions, rendering it a promising solar light-driven N2 to NH3 reduction (NRR) catalyst. The excellent formation energy of B doped C2N advocates its experimental synthesis.