We demonstrate an average ultraviolet (UV) power generation of 20 W by harmonically converting the output of an all-fiber master oscillator power amplifier (MOPA) system. The third-harmonic generation (THG) UV output provides a conversion efficiency of 40% from the amplified fundamental output. The seed source, which consists of a 1064-nm-wavelength continuous-wave laser diode and two cascaded intensity modulators, produces chirp-free pulses with tunable durations from 100 ps to 2 ns at arbitrary repetition rates and a high dynamic extinction ratio. The fiber MOPA system generates a maximum average power of 50 W and a maximum peak power of 83 kW.