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Promising materials for magnetic refrigeration and thermomagnetic power generation often display strong coupling between magnetism and structure. It has been previously proposed that MnCoP exhibits this strong coupling, contributing to its substantial magnetocaloric effect near TC = 578K. Here, we show from temperature-dependent synchrotron x-ray diffraction that MnCoP displays a discontinuity in the thermal expansion at TC, with spontaneous magnetostriction that is positive in the a direction and negative in the b direction, highlighting the anisotropic nature of the magnetostructural coupling. Varying the Mn:Co ratio of Mn2−xCoxP within the range of 0.6 ≤ x ≤ 1.4 allows the magnetic properties to be tuned. TC decreases as the composition deviates from stoichiometric MnCoP, as does the saturation magnetization. The magnitude of the magnetocaloric effect, |ΔSM|, decreases as well, due to broadening of the magnetic transition. The large reversible change in magnetization ΔM accessible over a small temperature range under moderate magnetic fields makes these materials promising for thermomagnetic power generation from waste heat.