Abstract: An isothermalized mirror assembly is a mirror structure that uses a heat pipe to isothermalize heat loads. The isothermalized mirror assembly includes at least one mirror unit and a quantity of thermally-convective fluid. The mirror unit includes a vacuum enclosure, a capillary medium, at least one reflector, and a plurality of cross supports. The vacuum enclosure is the structural base of the isothermalized mirror assembly and is used to retain a vacuum and the thermally-convective fluid. The cross supports are mounted within the vacuum enclosure and increases the structural integrity of the vacuum enclosure. The capillary medium is mounted across the interior of the vacuum enclosure and about each cross support. The capillary medium and the thermally-convective fluid work in conjunction to form a heat pipe within the vacuum enclosure. The reflector is externally mounted to the vacuum enclosure in order to redirect EM radiation.