In this study, we simultaneously introduced both poly(lactic acid) (PLA) and multiwalled carbon nanotubes (CNTs) into the polyurethane (PU) matrix via melt blending, to achieve balanced mechanical properties and good conductivity. Different contents of PLA (0–30 wt%) and CNTs (0–3.0 wt%) were used in this work. A significant improvement in tensile strength at 300% strain and Young's modulus were observed, which could not be obtained by incorporating either PLA or CNTs with PU separately. Particularly, the ternary composites containing a large amount of PLA (30 wt%), 70PU/30PLA/CNTs composites, exhibit superior mechanical properties compared to other composites with less PLA content but the same amounts of CNTs. Moreover, the ternary composites showed better electrical conductivity compared with the binary counterpart. SEM observations demonstrated that PLA particles and CNTs are separately dispersed in the PU matrix. It was found that PLA particles remain spherical and their size increases with increasing PLA content up to 30 wt%, while a network structure of CNTs is formed with increasing its content, which was also confirmed via dynamic rheological analysis. Interestingly, some CNTs were seen to be located in the interfaces between PLA particles and the PU matrix for 70PU/30PLA/CNTs composites, namely, some CNTs exhibit a nano-bridge effect between PU and PLA. We hypothesized that the nano-bridge structure of CNTs in the composites could mainly contribute to the observed enhancement of mechanical properties and electrical conductivity.