Effective performance of oil-based drilling fluids (OBFs) in demanding high-temperature environments hinges on the stability of their rheological properties. However, conventional organoclays (OCs) utilized to control these properties often exhibit thermal degradation at elevated temperatures, necessitating the development of more robust alternatives. Therefore, the present study aimed to develop a high-temperature-resistant OC to enhance OBF rheological stability. To address the thermal instability issues of conventional quaternary organo-montmorillonites (OMts), which manifest as interlayer structural collapse and particle aggregation, an innovative dual modification strategy was developed through the synergistic combination of quaternary ammonium intercalation and silane grafting. Montmorillonite (Mnt) was modified with dioctadecyl dimethyl ammonium chloride (1821) and stearyl dimethyl benzyl ammonium chloride (1827) at a mass ratio of 2:1 to yield 1821+1827-OMt, which was subsequently further modified with dodecyl trimethoxy silane (DTMS) to form 1821+1827+DTMS-OMt. Comparison of the properties of 1821+1827-OMt and 1821+1827+DTMS-OMt after high temperatures revealed the following. (1) Thermogravimetric (TG) and derivative thermogravimetric (DTG) analysis and gel volume tests demonstrated that the Si–O–Si bonds in 1821+1827+DTMS-OMt were thermally stable up to ~440°C, and the gel volume of the 1821+1827+DTMS-OMt suspension remained stable at 100 mL following high-temperature aging treatments. (2) X-ray diffraction and elemental analysis revealed that 1821+1827+DTMS-OMt exhibited a larger basal spacing and larger nitrogen content compared with ungrafted 1821+1827-OMt. (3) The suspension containing 1821+1827+DTMS-OMt demonstrated enhanced thermal stability at 260°C, evidenced by its narrower rheological parameter ranges of apparent viscosity (6–12 mPa s), plastic viscosity (5–9 mPa s), and yield point (1–3 Pa), compared with those of the suspension containing 1821+1827-OMt. (4) Optical microscopy demonstrated that 1821+1827+DTMS-OMt exhibited greater resistance to agglomeration at high temperatures than 1821+1827-OMt. The high-temperature-resistant OC developed in this study overcomes the thermal instability of traditional OCs, providing a robust approach for enhancing the efficiency and reliability of oil-based drilling fluids in high-temperature drilling applications.