Magnetic refrigeration, as well as waste-heat recovery, can be accomplished through the magnetocaloric effect, where temperature changes the magnetic state of a material or vice versa. Promising magnetocaloric materials display large changes in magnetic entropy (ΔSM) upon application of a moderate magnetic field and are often associated with magnetic materials possessing some degree of magnetostructural coupling. In such compounds, the magnetic transition is coupled to some structural transition at the ordering temperature, and indicators for these are readily calculated by the magnetic deformation proxy ΣM. MnSb, with a Curie temperature TC = 577 K, has a calculated magnetic deformation of ΣM = 5.9% and is a promising candidate material for waste-heat recovery. The temperature dependence of structural, magnetic, and magnetocaloric properties of Mn1+xSb, where x is a tunable amount of interstitial Mn, is studied here. Excess Mn is incorporated as an interstitial whose magnetic moment is antialigned with the stoichiometric Mn, and the excess Mn has the effect of lowering TC, such that the Curie temperature can be tuned from 577 K to nearly room temperature at 318 K for x = 0.2. For x = 0.0, 0.1, and 0.2, values of ΔSM under a maximum magnetic field H = 5 T are found to be 3.65, 3.00, and 2.83 J K−1 kg−1 , respectively. While the maximum ΔSM decreases with x, the high refrigerant capacitya more holistic measure of performanceis retained in this highly tunable system.