A mechanism for lightning initiation by the sustained runaway breakdown of air is presented. Unlike earlier models that rely upon large cosmic-ray air showers, this mechanism uses the runaway electrons produced by the steady background of atmospheric cosmic-rays to amplify non-uniformities in the electric field. The ionization of air from the runaway electrons creates a region of discharge that propagates in the opposite direction of the electrons, enhancing the electric field in front of it to the point where a conventional breakdown can occur. As the discharged region grows, positron feedback can become important, dramatically increasing the flux of runaway electrons and increasing the propagation speed of the discharged region up to 106 m/s. The ambient electric field strength needed for this mechanism is within the range of values observed in thunderclouds and is substantially lower than that required by most other mechanisms.