High power levels and high power densities associated with directed energy weapon systems, electronic warfare systems, and high thrust-to-weight aircraft propulsion systems require the development of effective and efficient thermal management solutions. As the objective for many high-power electronic systems is integration onto mobile platforms, strict requirements are also placed on the size, weight, and power draw of the corresponding thermal management system. High peak waste heat loads cannot be efficiently rejected to ambient air in a package integrated onto a mobile platform, leading to the need to store large amounts of energy in a compact, lightweight package. Thermal storage devices must not only be able to store energy rapidly at high power levels but they must also reject energy efficiently, allowing the thermal storage device to recharge for multiple uses. This paper will discuss the design of an advanced phase-change thermal storage device and present the effects of the storage device on system performance. Results of theoretical and experimental performance evaluations will also be presented. For this study, a model was developed and experimentally validated for determining heat rejection based on phase change material temperature and fluid temperature. For comparison, the heat rejection and thermal storage capabilities of a system using a 50%/50% by volume mixture of propylene glycol and water was established. The comparison shows how integrating the phase-change thermal storage device into a single-phase loop impacts the size, weight, and power draw of a system on a mobile platform. The thermal storage systems discussed in this paper are applicable to many different thermal management architectures, are easily adapted to meet the requirements of a wide range of high-power systems, and have potential to significantly reduce thermal management size, weight, and power requirements.