MoS2 is known to show stubborn n-type behavior due to its intrinsic band structure and Fermi level pinning. Here, we investigate the combined effects of molecular doping and contact engineering on the transport and contact properties of monolayer (ML) MoS2 devices. Significant p-type (hole-transport) behavior was only observed for chemically doped MoS2 devices with high work function palladium (Pd) contacts, while MoS2 devices with low work function metal contacts made from titanium showed ambipolar behavior with electron transport favored even after prolonged p-doping treatment. ML MoS2 transistors with Pd contacts exhibit effective hole mobilities of (2.3 ± 0.7) cm2 V-1 S-1 and an on/off ratio exceeding 106. We also show that p-doping can help to improve electrical contacts in p-type field-effect transistors: relatively low contact resistances of (482 ± 40) kΩ μm and a Schottky barrier height of ≈156 meV were obtained for ML MoS2 transistors. To demonstrate the potential application of 2D-based complementary electronic devices, a MoS2 inverter based on pristine (n-type) and p-doped monolayer MoS2 was fabricated. This work presents a simple and effective route for contact engineering, which enables the exploration and development of high-efficiency 2D-based semiconductor devices.