Mixed anion oxyhalides with the formula Ca2MnO3X (X = Cl, Br) are synthesized using solid-state reaction methods. These two materials crystallize in a novel structure type due to the small ionic radius of Ca and the strong Jahn-Teller effect of Mn3+. The resulting structure (space group Cmcm) contains one-dimensional chains of MnO4 square planes, with an angle of ∼120° between neighboring planes. At low temperatures, the two materials adopt magnetic arrangements, with ferromagnetic chains coupled antiferromagnetically. On applying a magnetic field, both materials experience spin-flop transitions.