The discovery of the electron predates the discovery of the proton.
The existence of electrons was first postulated by British physicist J.J. Thomson in 1897 during his experiments with cathode rays. Thomson observed that cathode rays could be deflected by electric and magnetic fields, indicating the presence of charged particles. He proposed that these particles were negatively charged and called them "corpuscles," which later came to be known as electrons.
On the other hand, the discovery of the proton came later. The proton was discovered by British physicist Ernest Rutherford in 1919 through his experiments on the scattering of alpha particles. Rutherford conducted experiments in which alpha particles (helium nuclei) were fired at a thin gold foil. He observed that some of the alpha particles were deflected by large angles, which led him to propose a model of the atom with a small, dense, positively charged nucleus. Rutherford named the positively charged particle in the nucleus the "proton."
So, the discovery of the electron preceded the discovery of the proton by a couple of decades.