The high temperature magnetization behavior of an air sintered nanocrystalline ZnO:Co (5 at. %) sample while it is subjected to thermal cycles in vacuum/air ambient is reported in the present paper. The pristine air sintered sample exhibits intrinsic room temperature ferromagnetism. Upon vacuum annealing, the magnetization of this sample increases significantly and the sample remains ferromagnetic up to 630 °C. This has been attributed to the formation of Co clusters, and thus, the ferromagnetic ordering is extrinsic in nature. The subsequent air annealing of the vacuum annealed ZnO:Co (5 at. %) sample, however, reverts back the magnetization to a value that was observed in the pristine air sintered sample. This is due to complete oxidation of the cobalt clusters. Furthermore, the sample’s magnetization at room temperature and also its temperature dependence up to 630 °C remain unaffected by further annealing in vacuum or air, a feature required of a material for its device applications. The Curie temperature of this robust sample is ∼450 °C.