Recently, studies of 2D organic layered materials with unique electronic properties have generated considerable interest in the research community. However, the development of organic materials with functional electrical transport properties is still needed. Here, a 2D fused aromatic network (FAN) structure with a C5N basal plane stoichiometry is designed and synthesized, and thin films are cast from C5N solution onto silicon dioxide substrates. Then field-effect transistors are fabricated using C5N thin flakes as the active layer in a bottom-gate top-contact configuration to characterize their electrical properties. The C5N thin flakes, isolated by polydimethylsiloxane stamping, exhibit ambipolar charge transport and extraordinarily high electron (996 cm2 V−1 s−1) and hole (501 cm2 V−1 s−1) mobilities, surpassing the performance of most pristine organic materials without doping. These results demonstrate their vast potential for applications in thin-film optoelectronic devices.