Promisingapplications in photonics are driven by the ability tofabricate crystal-quality metal thin films of controlled thicknessdown to a few nanometers. In particular, these materials exhibit ahighly nonlinear response to optical fields owing to the induced ultrafastelectron dynamics, which is however poorly understood on such mesoscopiclength scales. Here, we reveal a new mechanism that controls the nonlinearoptical response of thin metallic films, dominated by ultrafast electronicheat transport when the thickness is sufficiently small. By experimentallyand theoretically studying electronic transport in such materials,we explain the observed temporal evolution of photoluminescence intwo-pulse correlation measurements that we report for crystallinegold flakes. Incorporating a first-principles description of the electronicband structure, we model electronic transport and find that ultrafastthermal dynamics plays a pivotal role in determining the strengthand time-dependent characteristics of the nonlinear photoluminescencesignal, which is largely influenced by the distribution of hot electronsand holes, subject to diffusion across the film as well as relaxationto lattice modes. Our findings introduce conceptually novel elementsruling the nonlinear optical response of nanoscale materials, whilesuggesting additional ways to control and leverage hot carrier distributionsin metallic films.