In existing cementitious material systems, high strength and high ductility are often difficult to achieve simultaneously. High-strength cementitious materials typically rely on matrix densification to attain high compressive strength but exhibit pronounced brittleness, whereas strain-hardening cementitious composites achieve ductility mainly through fiber bridging and multiple cracking at relatively limited strength levels. To address this challenge, this study proposes a micromechanics-based design framework for high-strength, high-ductility cementitious composites (HSTCC). Building upon classical ECC theory, the framework integrates matrix densification, fiber network constraints, and fiber bridging micromechanics, and further formulates an energy-based strain-hardening criterion by linking fiber bridging energy with matrix fracture energy, enabling quantitative characterization of the conditions required for stable multiple cracking in high-strength cementitious matrices. Guided by the proposed framework, mix proportions satisfying coupled strength–ductility requirements were theoretically derived and validated through compressive, uniaxial tensile, and flexural tests conducted under different water-to-binder ratios. The results demonstrate that stable strain-hardening behavior with tensile strain capacity exceeding 3% can be achieved at compressive strengths above 120 MPa. These findings indicate that the proposed framework extends classical ECC micromechanics into high-strength matrix regimes and provides a physics-informed alternative to conventional empirical mix design approaches.