The scientist who first measured the charge of an electron was Robert A. Millikan. He performed his famous oil drop experiment between 1909 and 1917.

In this experiment, Millikan sprayed tiny oil droplets into a chamber and observed their motion as they fell through the air. By applying an electric field to the chamber, he could manipulate the droplets' motion and determine the charge on each droplet. He found that the charge on each droplet was always a multiple of a fundamental unit of charge, which he determined to be the charge of a single electron.