The cooling of hot electrons in graphene is the critical process underlying the operation of exciting new graphene-based optoelectronic and plasmonic devices, but the nature of this cooling is controversial. We extract the hot-electron cooling rate near the Fermi level by using graphene as a novel photothermal thermometer that measures the electron temperature (T(t)) as it cools dynamically. We find the photocurrent generated from graphene p-n junctions is well described by the energy dissipation rate CdT/dt = -A(T 3 -T l 3 ), where the heat capacity is C = αT and T l is the base lattice temperature. These results are in disagreement with predictions of electron-phonon emission in a disorder-free graphene system, but in excellent quantitative agreement with recent predictions of a disorder-enhanced supercollision cooling mechanism. We find that the supercollision model provides a complete and unified picture of energy loss near the Fermi level over the wide range of electronic (15 to ∼ 3,000 K) and lattice (10-295 K) temperatures investigated. © 2013 Macmillan Publishers Limited. All rights reserved.