  We present deep imaging of the ultra-diffuse Andromeda XIX dwarf galaxy from
the Advance Camera for Surveys on the Hubble Space Telescope which resolves its
stellar populations to below the oldest main sequence turn-off. We derive a
full star formation history for the galaxy using MATCH, and find no evidence of
star formation in the past 8 Gyr. We calculate a quenching time of
$\tau_{90}=9.7\pm0.2$~Gyr, suggesting Andromeda~XIX ceased forming stars very
early on. This early quenching, combined with its extremely large half-light
radius, low density dark matter halo and lower than expected metallicity make
it a unique galaxy within the Local Group and raises questions about how it
formed. The early quenching time allows us to rule out feedback from bursty
star formation as a means to explain its diffuse stellar population and low
density dark matter halo. We find that the extended stellar population, low
density halo and star formation could be explained by either tidal interactions
(such as tidal shocking) or by late dry mergers, with the latter also
explaining its low metallicity. Proper motions and detailed abundances would
allow us to distinguish between these two scenarios.
