Four isostructural metal monophosphonates, M{(2-C5H4NO)CH2PO3} (H2O)(2) with M = Co (1), Ni (2), Mn (3), and Cd (4), were synthesized and structurally characterized. These compounds show a double-chain structure in which the M-2(mu-O)(2) dimers are connected by O-P-O bridges. The magnetic responses of 1-3 were investigated over a wide range of magnetic fields (up to 10 T) and temperatures (down to 50 mK). Except for 4, which is weakly diamagnetic from 2 K to room temperature, the dominant magnetic interactions are antiferromagnetic. Isothermal magnetic field sweeps at 50 mK provide signatures in the magnetic responses that are associated with antiferromagnetic to field-induced fully polarized (magnetically saturated) transitions. Analysis of the magnetic data indicates that 1 and 2 form magnetic dimer-like clusters with weak dimer-dimer interactions present. Contrastingly, the magnetic interactions present in 3 are significantly weaker, so a definitive description of the magnetism of this compound is elusive.