The initial stages of defect generation in magnesia (MgO) single crystals irradiated with 1.2 MeV Au+ ions at 573, 773, and 1073 K and at different fluences have been studied. High-resolution X-ray diffraction was used to measure the irradiation-induced elastic strain. Point-defect relaxation volumes were computed using density functional theory calculations. The defect concentration was then calculated. It was found to increase with ion fluence at all temperatures, with maximum values being similar to 0.46 % at 573 K, similar to 0.24 % at 773 K, and similar to 0.13 % at 1073 K. The decrease in the maximum strain with increasing temperature indicates a dynamic annealing. The defect generation efficiencies were found to be very low and the values obtained were in the range of similar to 2.4, 1.2, and 0.6 % at 573, 773, and 1073 K, respectively. An annealing effect due to electronic energy deposition is suspected to explain these low values.