For advanced thermal interface materials (TIMs), massive inorganic addition for high isotropic thermal conductivities conflicts with suitable rheological viscosity for low contact thermal resistance. Traditional strategies rarely resolve such a contradiction, and it remains an academic and industrial challenge. Herein, inspired by the structure and function of the bone joint, a best‐of‐both‐worlds approach is reported that endows a standard polydimethylsiloxane/alumina (PDMS/Al2O3) TIM with simultaneously enhanced rheological mobility and thermal conductivity. It is conducted by employing morphology‐controllable gallium‐based liquid metal (LM) to the surface of Al2O3 by a scalable mechanochemical process. At the typical polymer‐LM‐Al2O3 interface, LM droplets with low cohesive energy can release the freedom for macromolecular chain relaxation and reduce the viscosity, successfully allowing the high‐loading TIMs (79 vol.%) to keep the thixotropic state and effectively reducing its contact thermal resistance with a copper substrate by 65%. At the same time, adjacent LMs merge to thermally bridge separate Al2O3 particles, which facilitates the interfacial thermal conduction and enhances the thermal conductivity from 5.9 to 6.7 W m−1 K−1. Along with additional electrical insulation, this filler modification strategy is believed to inspire others to develop high‐performance polymer‐based TIMs for future advanced electronics. A joint‐inspired interfacial engineering strategy is reported through on‐demand employing liquid metal to alumina by a scalable mechanochemical process, leading the original hard and non‐conductive polymer–alumina interface change into the liquid yet thermal conductive counterpart. It successfully resolves the traditional paradox that thermal conductivity and contact thermal resistance of a thermal interface material is hardly improved simultaneously.