The long-term stability of thermally conductive high-density polyethylene (HDPE)-based compounds as phase-change material (PCM) is investigated. For this purpose, the HDPE’s thermal conductivity (TC) is first enhanced via compounding two different filler types (expanded graphite and aluminum) into the polymeric matrix. Bulky specimens of these compounds are then stored in air for up to 7289 h in the melt state to investigate the compounds’ long-term stability as PCM. Their thermo-oxidative/thermal stability and their ability to maintain the isotropic material character (homogeneous distribution of the incorporated particles) is investigated. The compounds’ degradation behavior is monitored via Fourier-transform infrared spectroscopy (FTIR) and the maintenance of the homogeneous filler distribution is examined via a combined Differential Scanning Calorimetry (DSC)/Thermal Gravimetric Analysis (TGA) mapping of each exposed specimen. The storage capacity decreases minimally after 7289 h of exposure. Furthermore, the incorporated filler particles enhance the thermo-oxidative stability of HDPE as PCM. Consequently, thermally conductive HDPE is a highly interesting PCM.