An integrated lightweight structure and thermal insulation has been developed for the Mars Rover Warm Electronics Box (WEB). It combines a low thermal conductivity fiber reinforced composite structure with co-cured thermal control surfaces and an ultralightweight hydrophobic solid silica aerogel which eliminates convection and minimizes conduction and radiation. It provides excellent thermal insulation both at low gas pressures and in vacuum; and meets the structural requirements for spacecraft launch loads and for a 60 g impact landing at Mars without damage to the insulation or structure. The integrated design provides for variable conductance to meet the different thermal requirements for the different phases of operation. The system incorporates heat conduction paths to provide thermal control on the ground and during interplanetary cruise, which are then disconnected for Mars operation. The thermal performance of the design was evaluated at prototype level, and as a full structural engineering model and flight system. The thermal conductivity for the structure with insulation was measured at the prototype level to have a value of 0.033 W/mK and 24 °C at 1 atmosphere pressure, and in a10 torr CO2 simulated Mars environment to be 0.0163 W/mK at 24 °C, 0.0126 W/mK at -27 °C and 0.0089 W/mK at -80 °C.