The emerging production of ultrathin graphite material is applied to thermal management in a numerical comparison of aluminum and graphite-based plate-fin heat exchangers. Considering anisotropic thermal conductivity in which out-of-plane transport is about two orders of magnitude lower than in-plane values, the ultrathin graphite-based solution outperforms aluminum by rejecting up to 20% more heat on a volumetric basis. Thermal and hydraulic performance is characterized for both solutions over a range of airflow rates in a notional water/air device. Laminar through fully turbulent regimes are considered. Steady and unsteady three-dimensional (3-D) conjugate simulations reveal a faster equilibration rate for the ultrathin graphite-based solution, minimizing thermal lag that must be accounted for in on-demand electronics cooling. Fin optimization studies predict equivalent conductance with graphite at one-tenth the thickness of aluminum. The combination of improved heat rejection, rapid response rate, and low material density make an ultrathin graphite-based solution uniquely suited to aerospace thermal management.