We propose a methodology to infer the reheat temperature (TRH) of the Universe from the collider signal of freezing in dark matter (DM). We demonstrate it for the mono-γ signal at the electron-positron colliders, which indicates to a low-scale TRH, after addressing observed DM abundance, BBN, and other relevant constraints. The method can be used to correlate different reheating dynamics, DM models, and collider signals.