This work reports the mechanical strength and ionic conductivity of poly(methyl methacrylate) (PMMA)-based polymer gel electrolytes that are modified with interpenetrating networks. A series of poly(ethylene glycol dimethacrylate) (PEGDMA) polymers with different numbers of repeating ethylene oxide units is utilized as the network former. The mechanical strength of the electrolytes is estimated from their stress–strain curves. To assess ionic distribution in the gel electrolytes, an analysis is made of the infrared absorption bands of the symmetric stretching modes of SO3 in triflate anions that are incorporated as the lithium salt. The local viscosity around charge-carrying ions is estimated by means of the fluorescence depolarization method. Introduction of a PEGDMA network, in particular by modifying with lower molecular weight PEGDMA, is effective for increasing the mechanical strength of PMMA-based gel electrolytes. This feature can be explained by a cross-linking effect provided by the network former. In addition, the PEGDMA modification leads to an increase in ionic conductivity due to enhancement in both the free-ion fraction and the ionic mobility. This favorable feature probably results from the introduction of the ethylene oxide unit (as PEGDMA) which has a high donor number for Li+ ions and high chain flexibility. Consequently, the use of higher molecular weight PEGDMA is advantageous for conductivity enhancement.