Carbon nanotube yarns (CNTY) have the potential to serve as continuous reinforcement in functional-structural composite materials; however, there exists the insulation problem (or poor conductivity) of polymers that has hampered the development of CNT/polymer-based composite electronic devices. Herein, a strategy is proposed to develop CNT/polymer-based composite yarn with excellent electrical and mechanical properties by doping polyethyleneimine (PEI) polymer and then directional densification-stretching. The PEI polymer was well-infiltrated into intra- and inner CNT bundles to construct a consecutively conductive pathway, endowing an ultra-robust tensile strength (∼1207 MPa) and highly electrical conductivity (2.1 × 103 S cm−1) of composite yarn. The resulting CNT/PEI CYs also exhibit integrated performances of superior mechanical damping, anti-abrasive ability, extreme environment stability, and electrothermal properties. Additionally, they can act as stretchable conductors to maintain electrical stability during large-strain (100%) cyclic stretching. This engineering strategy provides novel insights into the development of continuous CNT/polymer-based composites for structural and functional composite materials.