Low concentration molecular doping of conjugated polymers is known to enhance electronic mobility while maintaining other device properties in organic field effect transistors (OFETs). Solution phase doping of dissolved conjugated polymers has been shown to reduce solubility and cause polymer aggregation. This has led most researchers to study the doping of conjugated polymers after spin coating in spite of adding additional processing steps. In this work, we probe the solution phase doping of poly(3-hexylthiophene) (P3HT) after colloidal self-assembly into nanofibers. P3HT nanofibers were found to remain colloidally stable even when doped at high molar ratios (1:1). The nanofiber morphology, crystallinity and optical properties were examined before and after doping and correlated to OFET performance characteristics. This work highlights a new methodology for the solution phase doping of self-assembled conjugated polymers.