Assuming the electron is at rest relative to the observer (me), it's mass is 9.10956 x 10-31 kilogram (kg) (for any electron since they are all the same)...
When an electron is accelerated to high speed, its mass increases because of relativistic effects. This increase is not significant until the speed of the particle, relative to the observer, becomes an appreciable fraction of the speed of light (c). If v is the speed of an electron in meters per second (m/s) and c is the speed of light (approximately 2.99792 x 108 m/s), then the mass of an electron in motion, me*, is:
me* = me / (1 - v2/c2)1/2 = 9.10956 x 10-31 / (1 - v2/c2)1/2 kg
(With partial thanks to Tech Target)