Detailed magnetic measurements and powder neutron diffraction studies have been performed on a three-dimensional (3D) oxalate-based metal–organic framework (NH4)5[Mn2Cr3(ox)9]·10H2O (ox2– = C2O42–). The thermal variations of the magnetic susceptibilitiy measured via field-cooled and zero-field-cooled procedures are compatible with an antiferromagnetic spin organization in the presence of some single-ion anisotropy. This is supported by the metamagnetic behavior observed at 0.7 T in the magnetization curve at 2 K. Temperature-dependent neutron diffraction experiments have been performed between 2.5 and 300 K on the powdered sample. The structural part of the neutron diffraction pattern matches with the C2/c space group found by single-crystal X-ray diffraction. Below 6 K, the long-range magnetic ordering leads to the observation of a single magnetic peak at 0.386 Å–1. The refinement of the neutron diffractogram allows to propose a magnetic structure where the magnetic moments of the manganese(II) and chromium(III) ions are noncollinear with a maximum angle of almost 70°. In agreement with an orbital analysis, the coupling between these magnetic moments bridged in a bis(bidentate) oxalate ligand is mainly ferromagnetic whereas it is mainly antiferromagnetic for metal ions bridged in a monodentate-bidentate mode.