The emergence of stretchable and wearable electronics raises a strong demand for stretchable electrolyte materials. Compared to the traditional liquid electrolyte, solid-polymer electrolytes (SPEs) are non-flammable and have improved mechanical and thermal stability, making them promising for applications in advanced stretchable electronics such as lithium batteries, supercapacitors, and light-emitting electrochemical cells (LECs). Current research activities in SPEs have focused on improving their electrochemical stability and ionic conductivity to replace flammable liquid electrolytes. Polymeric electrolytes are often complexes of lithium salts and high-molecular-weight polymers such as polyethylene oxide (PEO). However, these polymers typically have low conductivities at room temperature, typically ~ 0.01 mS/cm. Although adding plasticizers or liquid electrolyte into polymer electrolytes improves the ionic conductivity at room temperature, these composite electrolytes generally exhibit poor mechanical stretchability. For advanced stretchable electronics, mechanical performance becomes a critical performance consideration since SPEs need to tolerate repeated volume change during cycling and survive external forces during use. Thus, it remains challenging to fabricate SPE materials that have robust mechanical properties without sacrificing ionic conductivity. Here, we present a new stretchable SPE that shows both high mechanical performance and high ionic conductivity at room temperature using a mixture of an elastic graft copolymer and lithium triflate salt. This elastic graft copolymer has a butyl rubber backbone that provides mechanical flexibility, with PEO sides chains that complex lithium salts to provide ionic conductivity. The resulting SPE provides a superb stretchability of >1000% with an ionic conductivity of 0.01 mS/cm, which is comparable to that of PEO based SPEs. We demonstrate the application of this new stretchable SPE in iridium complex-based LECs. The high stretchability and ionic conductivity of the SPE provide stretchable and durable LECs that exhibit a negligibly changed peak radiance under repetitive strain.