Mg-doped graphitic carbon nitride (g-C3N4) composites were prepared using a facile thermal polymerization process under air atmosphere. The obtained materials can be employed as the solid base catalysts in Knoevenagel condensation reactions. The Mg-doped g-C3N4 composites show excellent performance as a solid catalyst and superior catalytic activity compared to the pure CN-U. The introduction of Mg species improved the overall basic property of CN-U, favoring Knoevenagel condensations. The 5MgCN-U catalyst exhibited high benzaldehyde conversion of 97.4% at 70 A degrees C and good cycling stability for four cycles, with 87.7% benzaldehyde conversion and nearly 100% benzylidene malononitrile selectivity. Additionally, the Mg-doped g-C3N4 composites from urea showed superior catalytic performance than those produced from dicyandiamide and melamine, indicating that the precursors contribute to the efficiency of Knoevenagel condensation reactions.