LaTiO₂N is modified by Mg doping to construct solid solution series, i.e., LaTi_1−xMg_xO_2+yN_1−y (0 ≤ x ≤ 0.5). This modification effectively decreases the defect concentration (particularly bulk Ti³⁺ species and anion vacancies), increases the surface hydrophilicity, positively shifts the valence band top albeit it also enlarges the bandgap and induces Ti/Mg ordering when x ≥ 0.2. These competitive tensions render an optimal composition at LaTi_0.9Mg_0.1O_2+yN_1−y (x = 0.1) where both a low defect concentration and a disordered state of Ti/Mg can be reached, demonstrating much-improved photocarrier separation and exceptional photocatalytic activity with apparent quantum efficiency as high as 13.02 % at 420 ± 20 nm for O₂-evolution. Overall water splitting (H₂/O₂ molar ratio = 2/1) with good recycling stability has been realized by integrating LaTi_0.9Mg_0.1O_2+yN_1−y into a Z-scheme system. These findings justify the efficacy of Mg as a useful modulator to improve the photocatalytic performance of metal oxynitrides.