Heteroatom-doped carbon nitride (CN) materials have shown much potential as metal-free photocatalysts for water splitting. Graphitic phosphorus-linked triazine network (g-PCN) materials are a unique class within this family of materials, but remain difficult to access due to long reaction times annealing at temperatures above 500 °C and often afford ill-understood structures. Here, we reveal a milder, lower temperature approach for the synthesis of catalytically active g-PCN materials through combining a room-temperature mechanochemical reaction of sodium phosphide and cyanuric chloride with brief (1 hour) annealing of the milled material at 300 °C. This rapid, low temperature procedure yields ordered g-PCN catalysts whose layered structure was determined through a combination of magic-angle spinning nuclear magnetic resonance (MAS NMR), X-ray photoelectron spectroscopy (XPS), powder X-ray diffraction (PXRD) and transmission electron microscopy (TEM). An excellent level of accuracy to simulated 31P MAS NMR signals and PXRD patterns were obtained for the structure of the synthesized layered phosphorus-linked triazine networks following dispersion-corrected density functional theory (DFT). The mechanochemically-generated g-PCN is a highly effective photocatalyst for the hydrogen evolution reaction, producing 122 µmol H2 h-1 g-1 under broad spectrum irradiation.