A simple approach to deposit multiwalled carbon nanotube (MWNT) networks onto glass fiber surfaces achieving semiconductive MWNT–glass fibers is reported, along with application of fiber/polymer interphases as in-situ multifunctional sensors. This approach demonstrates for the first time that the techniques of conducting electrical resistance measurements could be applicable to glass fibers for in situ sensing of strain and damage; the techniques were previously limited to conductive and semiconductive materials. The electrical properties of the single MWNT–glass fiber and the “unidirectional” fiber/epoxy composite show linear or nonlinear stress/strain, temperature, and relative humidity dependencies, which are capable of detecting piezoresistive effects as well as the local glass transition temperature. The unidirectional composites containing MWNT–glass fibers exhibit ultrahigh anisotropic electrical properties and an ultralow electrical percolation threshold. Based on this approach, the glass fiber—the most widely used reinforcement in composites globally—along with the surface electrical conductivity of MWNTs will stimulate and realize a broad range of multifunctional applications.