The mechanochemical, solvent‐free, highly regioselective, rhodium‐catalyzed C−H methylation of (hetero)arenes is reported. The reaction shows excellent functional‐group compatibility and is demonstrated to work for the late‐stage C−H methylation of biologically active compounds. The method requires no external heating and benefits from considerably shorter reaction times than previous solution‐based C−H methylation protocols. Additionally, the mechanochemical approach is shown to enable the efficient synthesis of organometallic complexes that are difficult to generate conventionally. Catalytic C−H methylation proceeds efficiently under solvent‐free, mechanochemical conditions, including the late‐stage functionalization of biologically active substrates and otherwise difficult‐to‐make metalacyclic complexes.