We discuss the applicability of magnesium tantalum oxynitride (in the ABO2N stoichiometry) as photocatalyst by calculating its fundamental properties from first principles, fully analyzing structural factors and material polymorphism. We find that, because of their favorable band edge nature, most of the configurations of the perovskite-type MgTaO2N are suitable for water splitting, while ilmenite-type MgTaO2N, the structurally expected more favored polymorph here investigated, is mostly unsuitable for the overall water splitting. Our results also show the strong correlation between the conduction band structures and octahedral tilting, i.e., the more tilted the octahedra, the larger the overlap between Ta 5d bands and anion 2p bands, with subsequent band gap opening and dispersion reduction. Therefore, in the perovskite-type MgTaO2N material design for photocatalytic applications, reduced (if not totally suppressed) octahedral tilting (i.e., via A-site doping) is highly desirable in order to improve the solar-to-energy performance of the material.