Semiconducting polymers have transport properties that can be tuned by both the synthetic design and processing techniques. Their thermopower, electronic conductivity, and low lattice thermal conductivity make them attractive in thermal management and thermoelectric applications, especially in form factors unfit for comparable inorganic ceramics. The effects of blending differing molecular weights and regioregularities of poly(3-hexylthiophene) (P3HT) on the thermal conductivity of films of P3HT doped with 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane were investigated in order to develop design rules for the co-optimization of thermal and electronic properties. The thermal conductivity of blended P3HT films at room temperature was found to be controlled by the distribution of molecular weights and regioregularities of chains. The resulting thermal conductivity of P3HT at room temperature was found to span 0.2–0.85 W/mK without specialized processing methods. Upon electrical doping, a significant decrease in thermal conductivity was found at all blending compositions despite each composition having a comparable electronic conductivity. These results suggest the blending of molecular weights and regioregularities as a rational means to optimize thermal conductivity while maintaining desired electronic properties in semiconducting polymers.