A high-performance naphthalene diimide (NDI)-based conjugated polymer for use as the active layer of n-channel organic field-effect transistors (OFETs) is reported. The solution-processable n-channel polymer is systematically designed and synthesized with an alternating structure of long alkyl substituted-NDI and thienylene–vinylene–thienylene units (PNDI-TVT). The material has a well-controlled molecular structure with an extended π-conjugated backbone, with no increase in the LUMO level, achieving a high mobility and highly ambient stable n-type OFET. The top-gate, bottom-contact device shows remarkably high electron charge-carrier mobility of up to 1.8 cm2 V−1 s−1 (Ion/Ioff = 106) with the commonly used polymer dielectric, poly(methyl methacrylate) (PMMA). Moreover, PNDI-TVT OFETs exhibit excellent air and operation stability. Such high device performance is attributed to improved π–π intermolecular interactions owing to the extended π-conjugation, apart from the improved crystallinity and highly interdigitated lamellar structure caused by the extended π–π backbone and long alkyl groups.