In this work, operationally and mechanically stable organic field‐effect transistors (OFETs) are demonstrated on aramid fiber‐based paper enabled by a simple and universal surface planarization method. By employing a nanoimprint lithography‐inspired surface smoothening method, rough aramid paper is successfully smoothened from a scale of several tens of micrometers to a sub‐nanometer‐scale surface roughness. Owing to the sub‐nanometer‐scale surface roughness of the aramid paper, the OFETs fabricated on the aramid paper exhibit decent field‐effect mobility (0.25 cm2 V−1 s−1) with a high current on‐to‐off ratio (>107), both of which are comparable with those of OFETs fabricated on rigid silicon substrates. Moreover, the OFETs fabricated on the aramid paper exhibit both high operational and mechanical stability; this is indicated by a bias‐stress‐induced threshold voltage shift (∆VTH ≈ 4.27 V under an excessive gate bias stress of 1.7 MV cm−1 for 1 h 30 min) comparable to that of OFETs on a rigid silicon substrate, moderate field‐effect mobility, and a threshold voltage stability under 1000 bending cycles with a compressive strain of 1%. The demonstration of highly stable OFETs on paper enabled by the simple planarization method will expand the potential use of various types of paper in electronic applications.