The development of sustainable photocatalytic systems capable of utilizing solar energy for simultaneous chemical production and environmental remediation has attracted considerable interest. In this study, a dual-functionalization-induced nitrogen vacancy engineered g-C3N4 homojunction photocatalyst (KNCN) was synthesized via sequential alkali-assisted thermal polymerization using KOH and NaOH. This functionalization strategy introduces abundant nitrogen vacancies and surface functional groups, generating defect-rich domains that form an internal homojunction within the carbon nitride framework. Structural and spectroscopic analyses reveal enhanced visible-light absorption, a narrowed band gap, and improved charge carrier dynamics due to efficient charge separation. The engineered KNCN photocatalyst exhibits significantly enhanced photocatalytic activity for multiple redox reactions under visible-light irradiation. The system achieves an ammonia production rate of 587 μmol L−1 h−1 during photocatalytic nitrogen fixation, which increases to 1452 μmol L−1 h−1 in the presence of methanol as a hole scavenger. Simultaneously, the catalyst promotes efficient photocatalytic oxygen reduction, generating hydrogen peroxide at a rate of 801 μmol L−1 h−1. The in situ-produced H2O2 further facilitates the oxidative degradation of tetracycline, achieving 93% removal within 70 minutes. The enhanced performance is attributed to the synergistic effects of nitrogen vacancy defects, improved oxygen activation, and S-scheme charge transfer within the homojunction structure.