Electrons were discovered by the British physicist J.J. Thomson. He was following the long puzzle of rays from a cathode. Based on his experiments, he made a bold speculation that those new charged particles are even smaller than atoms and could be the constituent of atoms, which were turned out to be correct.

There wasn't a single person who discovered it, but a series of people who discovered successively more about electrons over an extended period. Eugen Goldstein discovered that what we now call electrons can cast a shadow as particles or waves, and so could be considered the first person to recognize them as such. Hendrik Lorentz predicted the mass and charge of them, and the trio of J. J. Thompson, John Townson, and A. J. Wilson confirmed Lorentz' predictions. There were others, too, but these are the closest answers I can give to this (admittedly I am not an expert on particle physics).