We demonstrate cycle-slip-free electronic control of the carrier-envelope phase evolution of few-femtosecond light pulses, optical access to its sub-cycle fluctuations and suppression of the amplitude noise of a mode-locked laser by phase control. As a result, few-cycle light waveforms can now be synthesized with electric and magnetic fields reproducible to within a known phase error (<0.3 rad) and an unprecedentedly low amplitude error (<3%). The exploration of phase-sensitive light–matter interactions can be tackled.