The practical use of lithium metal anodes in solid-state batteries requires a conducting membrane with high lithium-ion conductivity, electrochemical stability, and mechanical strength. We report a facile one step in-situ synthesis and fabrication of high ion-conducting glass fiber (GF) reinforced, crosslinked poly(ethylene oxide) (cPEO) composite polymer electrolyte (CPE) with exceptionally high elastic modulus (up to 2.5 GPa) over a broad temperature range (20 °C – 245 °C). Such giant increase in mechanical strength origins from crosslinked PEO units bonded to the surface functional group of silica fibers through dynamic hydrogen and ionic bonding. High ionic conductivity is achieved by LiTFSI salt solvated in plasticized cPEO units where the anion (FSI-) units are tethered to PEO matrix and Li-ion favorably coordinated to the plasticizer (tertaglyme). Further, CPE with 10wt% plasticizer cycled in a Li-metal cell showed stable cycling more than 100 cycles for 4 months at 70 oC (1500 Coulombs/cm2 Li equivalents), without dendritic growth.. The GF reinforced CPE reported here has multifunctional use such as solid electrolytes for all solid-state batteries and membranes for redox-flow batteries. Although focus of this study is on lithium-based batteries, but the results are equally promising for other alkali metal cations such as sodium.