High-strength engineered cementitious composites (HS-ECCs) have garnered significant attention for their superior mechanical properties and ductility. However, their high autogenous shrinkage, caused by a low water-to-binder ratio, high cementitious content, and lack of coarse aggregate, often results in early-age cracking, limiting their broader use in civil engineering. Incorporating iron sand in HS-ECCs has enhanced their mechanical properties, reduced the carbon footprint, and moderately decreased shrinkage strain compared to traditional silica sand; however, the shrinkage strain remains substantial. This study aims to reduce the autogenous shrinkage of HS-ECCs further by incorporating united expanding admixtures (UEAs)—calcium oxide-based (CEA) and magnesium oxide-based (MEA) expansive agents—in varying amounts (3% to 10% by mass of cement). This study also examines the impact of these admixtures on the workability and mechanical properties of HS-ECCs. The results show that increasing the UEA content significantly reduces autogenous shrinkage strain, achieving a 40.66% reduction at 10% UEA, from 1007.31 με to 647.18 με. While higher UEA content decreases workability, as indicated by lower fluidity and penetration depth, the compressive strength remains largely unaffected. The tensile strength peaks at 12.38 MPa with 3% UEA but declines at higher contents, with higher UEA content effectively minimizing crack formation. The novelty of this research lies in the combined use of waste iron sand and UEA, optimizing the balance between workability, mechanical properties, and autogenous shrinkage reduction in HS-ECCs. These findings support the broader application of HS-ECCs in civil engineering projects requiring high mechanical properties and low shrinkage.