Developing material design guidelines for organic mixed ionic–electronic conductors (OMIECs) is critical to enable high efficacy mixed transport within bioelectronics. One important feature which has yet to be thoroughly explored is the role of molecular weight on OMIEC performance. In this work, we examined a series of prototypical glycolated polythiophene materials (P3MEEET) with systematically increasing molecular weights within organic electrochemical transistors (OECTs)─a common testbed for investigating mixed transport. We find that there is improved performance beyond an intermediate molecular weight; however, this relationship is electrolyte dependent. Operando analysis suggests that the enhanced mobility at higher molecular weights may be negated by significant swelling when operated in NaCl due to disruption of intercrystallite charge percolation. The role of molecular weight is revealed through operation in KTFSI, as doping occurs through cation expulsion, preventing detrimental swelling and maintaining percolative pathways. These findings demonstrate the importance of both molecular weight and electrolyte composition to enhance the performance of OMIECs.