Heptazine‐based conjugated polymeric carbon nitrides (PCNs) are promising metal‐free photocatalysts, yet their synthesis is challenging due to the electron‐deficiency and insolubility of heptazine units. Indeed, heptazine‐containing polymers have only been prepared through nucleophilic substitution with amines by using toxic cyameluric chloride as the starting material. Herein, we report the novel and environmentally friendly method for preparing heptazine‐based mesoporous PCNs with hydrazone links formed through a simple Schiff base condensation of melem‐NH2 and aldehydes. Unlike cyameluric chloride, melem‐NH2 is non‐toxic, stable, and can be readily obtained from melem and hydrazine in solution. We demonstrate that the hydrazone linkages and the heptazine units synergistically enhance the photocatalytic activity of PCNs in visible‐light‐driven aerobic oxidation of benzyl alcohol to benzaldehyde. In particular, the polymer constructed from melem‐NH2 and p‐phthalaldehyde shows 17 times more activity than graphitic carbon nitride (g‐C3N4).