Relativistic blazar jets and magnetized coronae of low-luminosity accretion flows, like Sgr A* at our Galactic Center, routinely display fast and bright flares of high-energy emission. Yet, the “engine” responsible for accelerating the emitting particles to ultra-relativistic energies is still unknown. We argue that magnetic reconnection — a process by which magnetic field lines of opposite polarity annihilate, releasing their energy to the particles — can satisfy all the basic conditions for the emission. In blazar jets, we show by means of fully-kinetic particle-in-cell (PIC) simulations, that reconnection can produce efficient dissipation, rough equipartition between electron and magnetic field energies, and extended non-thermal distributions of accelerated particles, as required by the observations. Reconnection can also explain the puzzling ultra-fast bright flares observed from a number of TeV blazars. In low-luminosity accretion flows, we show that reconnection can power both thermal and non-thermal emission, and we produce physically-grounded synthetic images and spectra to be compared with infrared and X-ray observations of Sgr A* and with the upcoming results of the Event Horizon Telescope.