Improving the environmental stability of conjugated polymers remains a fundamental challenge that limits their widespread adoption and commercial application in electronic and photonic devices. Although paraffin can have excellent barrier properties against moisture in ambient air, the use of conjugated polymer/paraffin blends to fabricate organic field-effect transistors (OFETs) with high environmental stability has not been attempted. Here, we demonstrate that conjugated polymer/paraffin blends can greatly enhance the environmental stability of OFETs. Compared to conventional systems such as poly(3-hexylthiophene) (P3HT)/polystyrene and P3HT/polydimethylsiloxane blends, P3HT/paraffin blends exhibit superior environmental stability after 30 days of exposure to the ambient atmosphere. Furthermore, the conjugated polymer/paraffin blends provide stable electronic properties under severe mechanical deformation [a strain (ε) of ∼150%], overcoming a critical challenge arising from the use of fragile crystalline conjugated polymer films for flexible and stretchable electronic devices. In comparison with a conventional spin-coating method, a shear-coating technique provides enhanced molecular ordering and alignment, resulting in improved charge carrier mobility in the blend film OFETs. In particular, shearing in the evaporation regime improves the molecular ordering and alignment of the blend films more than shearing in the Landau-Levich regime. Interestingly, the environmental stability of the sheared blend films varies depending on the shear speed. Specifically, OFETs based on blend films sheared at 0.5 and 6.0-10.0 mm s-1 exhibit excellent environmental stability, maintaining 80% of their initial mobility after 30 days of exposure to air. In contrast, the environmental stability of the OFETs decreases considerably when the blend films are sheared at 1.0-4.0 mm s-1; the mobility decreases to as low as ∼20% of the initial value.