Edward Emerson Barnard discovered the first comet via photographic means, D/1892 T1, on 13th October 1892. A gifted astronomer, Barnard is also known for discovering Barnard’s Star and being a pioneer of astrophotography.
FYI, a year before, Max Wolf, another pioneer of astrophotography, had discovered 323 Brucia, the first asteroid by photographic means.