In the region of hard photon energies, radiative leptonic decays represent important probes of the internal structure of hadrons. Moreover, radiative decays can provide independent determinations of Cabibbo-Kobayashi-Maskawa matrix elements with respect to purely leptonic or semileptonic channels. Prospects for a precise determination of leptonic decay rates with emission of a hard photon are particularly interesting, especially for the decays of heavy mesons for which currently only model-dependent predictions, based on QCD factorization and sum rules, are available to compare with existing experimental data. We present a non-perturbative lattice calculation of the structure-dependent form factors which contribute to the amplitudes for the radiative decays $H \to \ell \nu_\ell \gamma$, where H is a charged pseudoscalar meson. With moderate statistics, thanks to the use of improved estimators, we are able to provide rather precise, first-principles results for the form factors in the full kinematical (photon-energy) range. Our continuum-extrapolated lattice determinations may then be employed to compute the differential decay rate and the corresponding branching fraction and make comparisons with existing experimental data.