Cell-surface receptor complexes mediated by pro-inflammatory interleukin (IL)-12 and IL-23, both validated therapeutic targets, are incompletely understood due to the lack of structural insights into their complete extracellular assemblies. Furthermore, there is a paucity of structural details describing the IL-12-receptor interaction interfaces, in contrast to IL-23-receptor complexes. Here we report structures of fully assembled mouse IL-12/human IL-23-receptor complexes comprising the complete extracellular segments of the cognate receptors determined by electron cryo-microscopy. The structures reveal key commonalities but also surprisingly diverse features. Most notably, whereas IL-12 and IL-23 both utilize a conspicuously presented aromatic residue on their α-subunit as a hotspot to interact with the N-terminal Ig domain of their high-affinity receptors, only IL-12 juxtaposes receptor domains proximal to the cell membrane. Collectively, our findings will help to complete our understanding of cytokine-mediated assemblies of tall cytokine receptors and will enable a cytokine-specific interrogation of IL-12/IL-23 signaling in physiology and disease.