Metallic micrometamaterials exhibit exceptional specific strength and energy dissipation capacity, yet their mechanical behavior under extreme thermomechanical conditions remains poorly understood. Here, we uncover deformation mechanisms in metallic microlattices subjected to combined thermal and mechanical extremes. Copper microlattices were fabricated via localized electrodeposition process with submicron spatial resolution and, for the first time, compressed at cryogenic (-150°C) and room temperatures under high strain rates up to 100 s -1 . The copper microlattices, characterized by micron-sized grains and randomly oriented growth twins, exhibit distinct temperature and strain rate dependent deformation responses that lead to enhanced energy dissipation. Compression tests on copper micropillars, which are dimensionally equivalent the microlattice struts, reveal substantial shifts in deformation mechanisms from dislocation slip to mechanical twinning as a function of temperature and strain rate. Together, these results provide a comprehensive framework for designing metallic micrometamaterials optimized for extreme thermomechanical environments.