High-energy all-solid-state lithium (Li) batteries are of great promises as the next-generation energy storage devices. Among all choices of electrolytes, polymer-based systems have attracted wide-spread attention due to their low density, low cost, and excellent processability. However, they are generally mechanically too weak to effectively suppress Li dendrites, with relatively low ionic conductivity for reasonable kinetics at ambient temperature. In fact, there is a mechanical strength versus ionic conductivity dilemma in solid polymer electrolytes, where higher ionic conductivity generally brings about more mobile polymer chains and thus lower modulus. Herein, we developed for the first time a new design principle to mechanically reinforced composite polymer electrolyte (CPE) by introducing a stiff mesoporous backbone. As a result, over one order of magnitude higher modulus than the conventional polymer electrolytes (~0.43 GPa) and high ionic conductivity of ~0.6 mS cm-1 (30 °C) were simultaneously achieved. Furthermore, stable Li symmetric-cell cycling for >450 cycles and full cells with cathode areal capacity up to 2.1 mAh cm-2 were demonstrated. The mechanically reinforced CPE represents a new design methodology for solid-state electrolytes and offers opportunities for viable all-solid-state Li batteries.