Our meagre understanding of CFTR misfolding and its reversal by small-molecule correctors hampers the development of mechanism-based therapies of cystic fibrosis. Here we exploit a helical-hairpin construct—the simplest proxy of membrane-protein tertiary contacts—containing CFTR’s transmembrane helices 3 and 4 and its corresponding disease phenotypic mutant V232D to gain molecular-level insights into CFTR misfolding and drug rescue by the corrector Lumacaftor. Using a single-molecule FRET approach to study hairpin conformations in lipid bilayers, we find that the wild-type hairpin is well folded, whereas the V232D mutant assumes an open conformation in bilayer thicknesses mimicking the endoplasmic reticulum. Addition of Lumacaftor reverses the aberrant opening of the mutant hairpin to restore a compact state as in the wild type. The observed membrane escape of the V232D hairpin and its reversal by Lumacaftor complement cell-based analyses of the full-length protein, thereby providing in vivo and in vitro correlates of CFTR misfolding and drug-action mechanisms.