This paper investigates the use of multi-walled carbon nanotube/polypropylene (MWCNT/PP) films to enable ultrasonic welding and damage detection of glass fiber/polypropylene (GF/PP) composite joints in a single lap configuration. Three MWCNT concentrations above electrical percolation (15–25 wt%) were chosen to weld GF/PP adherends. The effect of MWCNT content on the welding process was evaluated by comparing weld quality through lap shear strength (LSS) tests and fractography analysis. Films containing 15 and 20 wt% MWCNT had no significant effect on the welding process, while 25 wt% reduced LSS by up to 39% compared to pure PP films due to their brittleness. Welded joints with MWCNT/PP films showed potential for damage detection through real-time electrical resistance changes at the interface. Thus, the multifunctional films proposed in this study represent a novel and promising approach for ultrasonic welding and damage detection of thermoplastic composite joints, but future work should focus on improving weld strength.