This study investigates the mechanical properties and durability of Limestone Calcined Clay Cement (LC3) in Engineered Cementitious Composites (LC3-ECC) reinforced with steel fibers, with contents varying from 0 to 2% by volume. The dynamic elastic modulus of the composites was determined using ultrasound and acoustic tests (Sonelastic), and a correlation between both tests was proposed. A 3-point bending test was conducted to determine the flexural strength, toughness, resilience, and ductility of the LC3-ECC. The compressive strength at 7 and 28 days was also evaluated. Finally, a durability test through chloride migration and a simplified carbon dioxide (CO2) emissions analysis was performed. Incorporating 2.0% fiber content resulted in increases of 40% in flexural strength, 1270% in toughness, 243% in resilience, and 166% in ductility. This demonstrates the ability of LC3-ECC to withstand strain under load without failure and to maintain residual stresses in the post-peak phase. The calculation of CO2 emissions indicated that incorporating fibers at percentages up to 2.0% increased the eco-efficiency to 7.89 MPa/kg CO2 compared to 6.78 MPa/kg CO2 for the composites without fibers in terms of flexural strength. Therefore, the study highlights that steel fibers balance between improving mechanical performance and minimizing environmental impact, particularly in the optimal 2% fiber mix that combines robust structural benefits with a manageable environmental footprint.