Organic electrochemical transistors (OECTs) exhibit strong potential for various applications in bioelectronics, especially as miniaturized, point‐of‐care biosensors, because of their efficient transducing ability. To date, however, the majority of reported OECTs have relied on p‐type (hole transporting) polymer mixed conductors, due to the limited number of n‐type (electron transporting) materials suitable for operation in aqueous electrolytes, and the low performance of those which exist. It is shown that a simple solvent‐engineering approach boosts the performance of OECTs comprising an n‐type, naphthalenediimide‐based copolymer in the channel. The addition of acetone, a rather bad solvent for the copolymer, in the chloroform‐based polymer solution leads to a three‐fold increase in OECT transconductance, as a result of the simultaneous increase in volumetric capacitance and electron mobility in the channel. The enhanced electrochemical activity of the polymer film allows high‐performance glucose sensors with a detection limit of 10 × 10−6 m of glucose and a dynamic range of more than eight orders of magnitude. The approach proposed introduces a new tool for concurrently improving the conduction of ionic and electronic charge carriers in polymer mixed conductors, which can be utilized for a number of bioelectronic applications relying on efficient OECT operation.