An exploration of the manganese oxalate-phosphoric acid-water system under hydrothermal conditions, and using "reagent" quantities only of water, has led to the isolation of a new mixed anion framework material Mn-2(H2PO4)(2)(C2O4). This material features continuous chains of cis edge-sharing MnO6 octahedra, a motif which is unique among mixed phosphate-oxalate materials identified so far. These octahedral chains are linked into a three-dimensional framework via corner-sharing with H2PO4 tetrahedra, with oxalate ions acting as a bis-bidentate ligand in the third direction. Magnetic susceptibility studies show that this material may be modeled as an antiferromagnetic, S = 5/2 Heisenberg chain, with weaker coupling between the chains.