Despite extensive advancements in Engineered Cementitious Composites (ECCs), mixture design remains predominantly empirical, due to the absence of a quantitative parameter directly linking fiber–matrix interfacial mechanics to strain-hardening performance. This study identifies fiber–matrix interfacial friction as a quantifiable parameter and establishes a micromechanics-guided interfacial regulation framework to enhance the toughness of ECC by regulating fiber failure modes. First, a critical fiber–matrix interfacial frictional stress, (τ₀)_crit, corresponding to the transition between fiber pull-out and fracture, was theoretically derived based on energy dissipation maximization during crack propagation. A back-calculation approach was further developed to determine interfacial frictional stress (τ₀) directly from tensile stress–crack opening responses under single-crack tension, eliminating reliance on single-fiber pull-out testing. Then, τ₀ was tuned toward (τ₀)_crit through interfacial regulation using fly ash. Experimental results demonstrate that the toughness of ECC is maximized when τ₀ approaches (τ₀)_crit, confirming the validity of the proposed toughness enhancement mechanism. The study establishes an explicit mechanistic linkage between interfacial micromechanics and macroscopic strain-hardening performance, providing a predictive and quantitative design pathway that transcends empirical mixture adjustment.