Two-dimensional transition metal dichalcogenide (TMD) materials, albeit promising candidates for applications in electronics and optoelectronics(1-3), are still limited by their low electrical mobility under ambient conditions. Efforts to improve device performance through a variety of routes, such as modification of contact metals(4) and gate dielectrics(5-9) or encapsulation in hexagonal boron nitride(10), have yielded limited success at room temperature. Here, we report a large increase in the performance of TMD field-effect transistors operating under ambient conditions, achieved by engineering the substrate's surface morphology. For MoS2 transistors fabricated on crested substrates, we observed an almost two orders of magnitude increase in carrier mobility compared to standard devices, as well as very high saturation currents. The mechanical strain in TMDs has been predicted to boost carrier mobility(11), and has been shown to influence the local bandgap(12,13) and quantum emission properties(14) of TMDs. With comprehensive investigation of different dielectric environments and morphologies, we demonstrate that the substrate's increased corrugation, with its resulting strain field, is the dominant factor driving performance enhancement. This strategy is universally valid for other semiconducting TMD materials, either p-doped or n-doped, opening them up for applications in heterogeneous integrated electronics.