The synthesis, structure, and magnetic properties of a polar and magnetic oxynitride MnTaO2N are reported. High-pressure synthesis at 6GPa and 1400 degrees C allows for the stabilization of a high-density structure containing middle-to-late transition metals. Synchrotron X-ray and neutron diffraction studies revealed that MnTaO2N adopts the LiNbO3-type structure, with a random distribution of O2- and N3- anions. MnTaO2N with an orbital-inactive Mn2+ ion (d(5); S=5/2) exhibits a nontrivial helical spin order at 25K with a propagation vector of [0,0,] (approximate to 0.3), which is different from the conventional G-type order observed in other orbital-inactive perovskite oxides and LiNbO3-type oxides. This result suggests the presence of strong frustration because of the heavily tilted MnO4N2 octahedral network combined with the mixed O2-/N3- species that results in a distribution of (super)-superexchange interactions.