A new measurement of the electron magnetic moment, g/2, has been performed with an uncertainty 2.7 and 15 times smaller than previous measurements from 2006 and 1987. The result is g/2 = 1.00115965218073(28) [0.28 ppt]. This measurement, combined with quantum electrodynamics (QED) theory, determines the fine structure constant α⁻¹ = 137.035999084(51) [0.37 ppb], with an uncertainty 20 times smaller than any independent determination of α. The electron magnetic moment, μ, is a measurable property of the electron, revealing its interaction with the fluctuating QED vacuum and probing for size or composite structure. The measurement of g/2, the magnitude of μ scaled by the Bohr magneton, is crucial for testing QED and probing for new physics beyond the standard model. The new measurement improves the accuracy of the fine structure constant, allowing for more precise tests of QED and the search for low-mass dark matter particles. The measurement uses a cylindrical Penning trap cavity to confine a single electron and inhibit spontaneous emission. Quantum jump spectroscopy and quantum non-demolition (QND) measurements are used to determine the cyclotron and spin frequencies. The electron is also used as a magnetometer to accumulate quantum-jump lineshape statistics over days, enabling the comparison of methods for extracting resonance frequencies. The new measurement and updated QED theory determine α with an uncertainty 20 times smaller than any independent method. The accuracy of the new g/2 value sets the stage for improved CPT tests with leptons and allows for more precise tests of QED. The measurement also contributes to the search for low-mass dark matter particles if a more accurate independent measurement of α becomes available. The measurement is 15 times more accurate than the 1987 measurement and 2.7 times more accurate than the 2006 measurement. The results are significant for testing QED, probing for electron size, and searching for low-mass dark matter particles. The measurement is supported by the NSF AMO program.A new measurement of the electron magnetic moment, g/2, has been performed with an uncertainty 2.7 and 15 times smaller than previous measurements from 2006 and 1987. The result is g/2 = 1.00115965218073(28) [0.28 ppt]. This measurement, combined with quantum electrodynamics (QED) theory, determines the fine structure constant α⁻¹ = 137.035999084(51) [0.37 ppb], with an uncertainty 20 times smaller than any independent determination of α. The electron magnetic moment, μ, is a measurable property of the electron, revealing its interaction with the fluctuating QED vacuum and probing for size or composite structure. The measurement of g/2, the magnitude of μ scaled by the Bohr magneton, is crucial for testing QED and probing for new physics beyond the standard model. The new measurement improves the accuracy of the fine structure constant, allowing for more precise tests of QED and the search for low-mass dark matter particles. The measurement uses a cylindrical Penning trap cavity to confine a single electron and inhibit spontaneous emission. Quantum jump spectroscopy and quantum non-demolition (QND) measurements are used to determine the cyclotron and spin frequencies. The electron is also used as a magnetometer to accumulate quantum-jump lineshape statistics over days, enabling the comparison of methods for extracting resonance frequencies. The new measurement and updated QED theory determine α with an uncertainty 20 times smaller than any independent method. The accuracy of the new g/2 value sets the stage for improved CPT tests with leptons and allows for more precise tests of QED. The measurement also contributes to the search for low-mass dark matter particles if a more accurate independent measurement of α becomes available. The measurement is 15 times more accurate than the 1987 measurement and 2.7 times more accurate than the 2006 measurement. The results are significant for testing QED, probing for electron size, and searching for low-mass dark matter particles. The measurement is supported by the NSF AMO program.