Abstract The αβ TCR has recently been suggested to function as an anisotropic mechanosensor during immune surveillance, converting mechanical energy into a biochemical signal upon specific peptide/MHC ligation of the αβ clonotype. The heterodimeric CD3εγ and CD3εδ subunits, each composed of two Ig-like ectodomains, form unique side-to-side hydrophobic interfaces involving their paired G-strands, rigid connectors to their respective transmembrane segments. Those dimers are laterally disposed relative to the αβ heterodimer within the TCR complex. In this paper, using structure-guided mutational analysis, we investigate the functional consequences of a striking asymmetry in CD3γ and CD3δ G-strand geometries impacting ectodomain shape. The uniquely kinked conformation of the CD3γ G-strand is crucial for maximizing Ag-triggered TCR activation and surface TCR assembly/expression, offering a geometry to accommodate juxtaposition of CD3γ and TCR β ectodomains and foster quaternary change that cannot be replaced by the isologous CD3δ subunit’s extracellular region. TCRβ and CD3 subunit protein sequence analyses among Gnathostomata species show that the Cβ FG loop and CD3γ subunit coevolved, consistent with this notion. Furthermore, restoration of T cell activation and development in CD3γ−/− mouse T lineage cells by interspecies replacement can be rationalized from structural insights on the topology of chimeric mouse/human CD3εδ dimers. Most importantly, our findings imply that CD3γ and CD3δ evolved from a common precursor gene to optimize peptide/MHC-triggered αβ TCR activation.