The nanoscale localizations, interactions, and conformations of endocytic proteins are key regulators of clathrin-mediated endocytosis. Although super-resolution microscopy has revealed the detailed organization of endocytic proteins, the resolution of super-resolution imaging cannot assess molecular interaction and conformational changes. Fluorescence resonance energy transfer (FRET) occurs when dyes are separated by less than 10 nm for most fluorescent proteins pairs. Thus, it can be used to map close-range molecular complexes and dynamics. Yet, to understand how endocytosis works, measurements from FRET must be correlated to the distinct stages of endocytosis. To accomplish this, we developed a new correlative lifetime-based FRET (FLIM-FRET) and platinum replica transmission electron microscopy (PREM) method, named FRET-CLEM. Here, FRET-based atomic distances can be mapped directly to individual cellular structures visualized in EM at the plasma membrane. We used this method to measure the conformational changes in clathrin light chain (CLC), a component of the clathrin triskelion and assembled clathrin lattice. CLC conformational changes have been proposed to regulate the assembly of clathrin in solution. However, CLC conformational changes and their effects on clathrin lattice growth, curvature, and endocytosis at the membrane of living cells are unknown. Using FRET-CLEM, we discovered that CLC undergoes a conformational switch as clathrin lattices curve. Preventing this conformational switch with acute chemical tools increased clathrin lattice sizes and inhibited endocytosis. Therefore, a specific conformational switch in CLC regulates lattice curvature and endocytosis in mammalian cells. These new correlative light and EM data will help develop a complete mechanistic model of endocytosis. More generally, FRET-CLEM can map molecular interactions and conformational changes at targeted membrane-associated proteins at identified cellular compartments including exocytic sites and neuronal synapses.