Well, J.J. Thompson was the first to discover the existence of a negatively charged particle, which turned out to be the Electron. So, to answer your question, it was J.J. Thompson. Also note that JJ thought that de electrons where INSIDE the atom and envelloped in a positively charged layer. P.S. It was 1st called a corpuscle by Thompson and was later changed by another scientist to electron.
The experiment that was used to discover the electron was the cathode ray experiment performed by JJ Thomson. In the presence of a magnetic field, cathode rays were observed to bend. When the poles were reversed, the ray bent the opposite direction. This let JJ Thomson deduce that they were actually negatively charged particles.
Look the atom was first drawn by John Dalton which was like a shell. JJ Thompson discovered it by conducting Thomas Alva Edison's Cathode Ray experiment. He suggested that the atom was covered with electrons like pimples on someones face. Since John Dalton suggested that the atom was electrically neutral JJ Thompson proposed that The atom was positively charged with the negatively charged electron on the body of the atom. Like Pimples.
Thompson's model described that every atom had an electron and he tried to incorporate the electron into the model of the atom... He prooved the Greeks wrong because they initially thought that the atom was solid.(This also prooved Dalton to be incorrect also) All in all JJ Thompson discovered the electron!