The outer membrane (OM) of Gram-negative bacteria is an asymmetric lipid bilayer with outer leaflet lipopolysaccharides (LPS) exposed to extracellular milieu and inner leaflet phospholipids (PLs) facing the periplasm. This unique lipid asymmetry is the key to its innate drug resistance, rendering the OM impermeable to external insults, including antibiotics and bile salts. To maintain this OM barrier, the OmpC-Mla system removes mislocalized PLs from the OM outer leaflet, and transports them back to the inner membrane (IM); in the first step, the OM OmpC-MlaA complex transfers PLs to the periplasmic chaperone MlaC. This process likely occurs via a hydrophilic channel in MlaA, yet mechanistic details have remained elusive. Here, we biochemically and structurally characterize the architecture of the MlaA-MlaC transient complex. We map the interaction surfaces between MlaA and MlaC in Escherichia coli, revealing that MlaC binds MlaA at the periplasmic face in a manner that juxtaposes the MlaA channel and the MlaC lipid binding cavity. In addition, we show that electrostatic interactions between the putative C-terminal tail helix of MlaA and a surface patch on MlaC are important for recruitment of the latter to the OM. We further provide biochemical evidence for conformational changes in the MlaA channel that correlate with interactions with MlaC and OM porins, as well as functional states of MlaA. Finally, we solve the cryo-EM structure of OmpC-MlaA in nanodiscs in a complex with MlaC, highlighting membrane thinning as a plausible mechanism for directing lipids into the MlaA channel. Our work offers critical insights into how the OmpC-MlaA complex catalyzes retrograde transport of PLs to the IM to maintain OM lipid asymmetry.