The work of Robert Millikan and Joseph John Thomson was most important in determining the mass of an electron.

Here's how:

* J.J. Thomson discovered the electron in 1897 through his cathode ray tube experiments. He determined the charge-to-mass ratio (e/m) of the electron.

* Robert Millikan conducted his famous oil drop experiment in 1909. He measured the charge of a single electron with great precision.

By combining Thomson's e/m value and Millikan's charge (e), scientists were able to calculate the mass (m) of an electron using the formula:

m = e / (e/m)

This groundbreaking work provided the first accurate measurement of the electron's mass, a fundamental constant in physics.