A laser system thermal management system includes a laser gain assembly and a thermal management assembly. The laser gain assembly includes a laser gain medium and may include laser pump diodes. The thermal management system includes a high pressure gas tank connected to an open-cycle Joule-Thompson refrigerator. Cooled and partially liquefied gas is introduced into a reservoir. The reservoir may be in good direct thermal contact with the laser gain assembly or via a closed loop recirculating fluid heat exchanger. The heat generated by the laser gain assembly is removed by heat exchange with the cooled gas and condensate in the reservoir either by direct thermal contact or via the recirculating heat exchanger loop. Gas evaporating in the reservoir is vented.