In the recent decade, polymer thermoelectric (TE) composite has witnessed explosive achievements to address energy generation and utilization. Besides the significant progress in enhancement of TE performance, the high mechanical property has received increasing attention, being important for practical applications in complex environments. However, the mechanical performance has always been improved at the sacrifice of TE performance, and vice versa, which poses a great challenge. Here, ionic liquid (IL)-assisted fabrication of flexible films of polymer TE composites with simultaneously high TE and mechanical performances based on poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), polyvinyl alcohol (PVA), and single-walled carbon nanotubes (SWCNTs) are reported. The resultant composite shows a high TE performance with a power factor of 106.1 ± 8.2 µW m K at room temperature, and strong mechanical robustness with a tensile modulus of 4.2 ± 0.5 GPa and fracture strength of 136.5 ± 10.6 MPa. It is the most mechanically robust TE composite known with such a high power factor in the available literature. The present study provides a promising way to help address the longstanding and intractable issue of inferior mechanical performance of TE composites without compromising TE performance.