Semiconducting self-assembled monolayers (SAMs) represent highly relevant components for the fabrication of organic thin-film electronics because they enable the precise formation of active it-conjugates in terms of orientation and layer thickness. In this work, we demonstrate self-assembled monolayer field-effect transistors (SAMFETs) composed of phosphonic acid oligomers of 3-hexylthiophene (oligothiophenes-OT) with systematic variations of thiophene repeating units (5, 10, and 20). The devices exhibit stable lateral charge transport with increased mobility as a function of thiophene unit counts. Importantly, our work reveals the packing and intermolecular order of varied-chain-length SAMs at the molecular scale via X-ray reflectivity (XRR) and quantitative X-ray photoelectron spectroscopy (XPS). Short oligomers (OT5-PA and OT10-PA) arrange almost perpendicular to the substrate, forming highly ordered SAMs, whereas the long-chain OT20-PA exhibits a folded structure. By tuning the molecular order in the monolayers via the SAM substitution reaction, the OT20-PA devices show a tripling in mobility.