In order to realize many devices based on semiconductor nanowires, reliable doping is essential. For such devices, it is important that the electron mobility is not compromised by doping incorporation. Here, we show that core-shell GaAs/AlGaAs nanowires can be modulation n-type doped with negligible loss of electron mobility. Optical pump terahertz probe spectroscopy is used as a novel, reliable, noncontact method of determining the doping density, carrier mobility and charge carrier lifetimes for these n-type nanowires and an undoped reference. A carrier concentration of 1.10 ± 0.06 × 10 ¹⁶ cm ^-3 was extracted proving the effectiveness of modulation doping in GaAs nanowires. The room-temperature electron mobility was found to be high at 2200 ± 300 cm ² V ^-1 s ^-1 with no degradation in comparison to undoped reference nanowires. In addition, modulation doping was found to enhance both the photoconductivity and photoluminescence lifetimes to 3.9 ± 0.3ns and 2.4 ± 0.1ns respectively, revealing that modulation doping can passivate interfacial trap states. ¹