This study investigates the early-age (7 days) tensile stress relaxation and cracking behavior of slag- and fly ash-based strain-hardening cementitious composites (SHCCs) under sealed curing. Mechanical properties were characterized via compressive strength, Young’s modulus, and uniaxial tensile tests, while stress relaxation was assessed under both pre-cracking (70% and 90% of the first cracking stress) and post-cracking (0.6% and 1.0% tensile strain) conditions. Crack formation and evolution were monitored using digital image correlation (DIC). Compared to fly ash-based SHCCs, slag-based SHCCs exhibited higher compressive strength and stiffness due to faster hydration and the development of a denser, more viscoelastic matrix. Both mixtures displayed ductile, strain-hardening behavior; however, slag-based SHCCs showed slightly superior tensile performance with higher strength and finer multiple cracking. Stress relaxation in slag mixes was more pronounced under both pre- and post-cracking conditions, particularly at higher stress levels, likely due to enhanced microcrack formation and fiber debonding. In contrast, fly ash-based SHCCs exhibited greater relaxation at lower stress levels. For both materials, the most significant stress decay occurred within the first 30 min, followed by a slower, stable phase. DIC analysis confirmed minimal change in existing crack geometry during relaxation, but revealed the initiation of new microcracks, especially in slag-based specimens, contributing to greater deformation and stress decay. These findings enhance the understanding of time-dependent tensile behavior in SHCCs and support their application in structural repair systems requiring long-term deformation control.