Incorporating carbon nanomaterials like carbon nanotubes (CNTs), reduced graphene oxide (rGO), and bucky paper into fiber-reinforced polymer composites (FRPCs) has gained interest for enhancing strength and adding piezoresistivity. These advancements offer alternatives for structural health monitoring, overcoming limitations of conventional sensors like added weight, manufacturing complexity, and risks to structural integrity. Investigating the in-plane properties and time-dependent piezoresistivity of multifunctional composites is essential for understanding material behavior under stress and enhancing their design and functionality. This foundational knowledge allows for the development of models that improve next-generation composites’ suitability for a wide range of applications. In this work, the manufacturing of unidirectional (UD) glass fiber/epoxy multifunctional composites is carried out using the vacuum assisted resin transfer molding (VARTM) process with embedding a layer of rGO-coated UD glass fabric at the mid-plane as an integrated sensor. First, tensile tests were conducted to determine the longitudinal mechanical and piezoresistive properties. During these experiments, the tensile load was applied in the fiber direction and the fractional change of resistance (FCR) was measured both in the longitudinal and transverse directions. In-plane properties showed a higher longitudinal gauge factor than transverse, as fibers compressed in the transverse direction due to the Poisson effect. Then, the stress relaxation tests were completed by keeping a constant tensile strain of 0.45% and monitoring the changes in electrical resistance for a duration of two hours upon the stress relaxation. During stress relaxation, FCR decreased due to enhanced network connectivity, indicating effective stress translation into FCR changes.