This thesis presents the first isotope-shift measurement of bound-electron g-factors of highly charged ions and determines the most precise value of the electron mass in atomic mass units, which exceeds the value in the literature by a factor of 13. As the lightest fundamental massive particle, the electron is one of nature’s few central building blocks. A precise knowledge of its intrinsic properties, such as its mass, is mandatory for the most accurate tests in physics – the Quantum Electrodynamics tests that describe one of the four established fundamental interactions in the universe. The underlying measurement principle combines a high-precision measurement of the Larmor-to-cyclotron frequency ratio on a single hydrogen-like carbon ion studied in a Penning trap with very accurate calculations of the so-called bound-electron g-factor. For the isotope-shift measurement, the bound-electron g-factors of two lithium-like calcium isotopes have been measured with relative uncertainties of a few 10^, constituting an as yet unrivaled level of precision for lithium-like ions.