TCRs recognize cognate pMHCs to initiate T cell signaling and adaptive immunity. Mechanical force strengthens TCR-pMHC interactions to elicit agonist-specific catch bonds to trigger TCR signaling, but the underlying dynamic structural mechanism is unclear. We combined steered molecular dynamics (SMD) simulation, single-molecule biophysical approaches, and functional assays to collectively demonstrate that mechanical force induces conformational changes in pMHCs to enhance pre-existing contacts and activates new interactions at the TCR-pMHC binding interface to resist bond dissociation under force, resulting in TCR-pMHC catch bonds and T cell activation. Intriguingly, cancer-associated somatic mutations in HLA-A2 that may restrict these conformational changes suppressed TCR-pMHC catch bonds. Structural analysis also indicated that HLA polymorphism might alter the equilibrium of these conformational changes. Our findings not only reveal critical roles of force-induced conformational changes in pMHCs for activating TCR-pMHC catch bonds but also have implications for T cell-based immunotherapy. Display omitted •Force-enhanced peptide-TCR interactions determine TCR-pMHC-I catch bonds•Force-strengthened TCR-MHC binding interface contributes to TCR-pMHC-I catch bonds•Force-induced MHC-I rotation allosterically enhances TCR-pMHC-I catch bonds•Tumor-associated somatic mutations in HLA-A2 impair TCR-pHLA-A2 catch bonds Wu et al. report that a dynamic structural mechanism of mechano-chemical coupling for TCR antigen recognition—that is, mechanical force-induced conformational changes in the agonist peptide-MHC-I—allosterically activates TCR-pMHC-I catch bonds to determine TCR antigen recognition and trigger T cell signaling.