This study investigates the development and evaluation of CNT-doped thermoplastic filament sensors for in-situ monitoring of composite structures. The piezoresistive behavior of filaments is first characterized under tensile loading by varying CNT content and sensing length to identify a suitable sensor configuration. A configuration with 4 wt% CNT content and a sensing length of 200 mm was selected based on its stable and reproducible response. The selected sensor is then placed at different laminate locations and evaluated under static three-point bending, using conventional electrical resistance strain gauges as a benchmark. The influence of plasma surface treatment on mechanical and sensing performance is also examined, and sensor robustness is assessed through cyclic bending and accelerated weathering. Static tests show that sensor placement has a negligible effect on laminate flexural modulus, with differences below 1% between embedded and surface-mounted configurations. Plasma-treated specimens exhibit increased stiffness but reduced piezoresistive linearity. The sensors exhibit linear and repeatable responses with gauge factors of 0.3–0.4 in the low-strain regime (0–0.3%). Under cyclic loading (2000 cycles), the sensors stabilize after an initial conditioning phase and maintain repeatable responses. Weathering induces a baseline resistance shift (up to ∼6%) without degrading sensing functionality after reconditioning.Overall, the results demonstrate that CNT-doped filaments are suitable as embedded strain sensors for structural health monitoring.