Microcrystal electron diffraction (MicroED) is a cryogenic electron microscopy (cryo-EM) method that determines protein structures from submicron crystals. G protein–coupled receptors (GPCRs) are membrane proteins that are critically important drug targets. These proteins require crystallization in lipidic cubic phase (LCP), making standard MicroED approaches intractable for investigating these samples. Here, we show that GPCR microcrystals grown in an LCP can be made amenable for MicroED by converting the LCP to the sponge phase and then ion-beam milling the crystals into thin lamellae. Our findings provide the basis for solving GPCR structures using MicroED, with future applications in structure-based drug discovery.