This study explores the effects of accelerated CO2 curing on the physical, mechanical, and microstructural characteristics of accelerated cured steel slag (ACCS) mortar. The mortar was formulated using basic oxygen furnace (BOF) slag, combined with 4.5 mm down fine aggregates and a chemical admixture. The mortar specimens were demolded 18 h after casting and then placed in a carbonation chamber for 24 h to undergo carbonation curing. Following the curing process, the mortar samples were subjected to various tests. To identify an optimal water-cement (w/c) ratio, workability was assessed through flow table tests across 20 different mix proportions. CO2 curing was applied to the test samples. The findings indicated that CO2 curing shortened the setting and curing times of the ACCS mortar, increased carbonation depth from 2 to 20 mm, improved mechanical properties by 3–14 MPa, reduced water absorption by 20%, and enhanced mortar density. A field emission-scanning electron microscope (FE-SEM) analysis and X-ray diffraction (XRD) test demonstrated that the ACCS-mortar matrix was denser than non-carbonated mortar specimens without BOF slag. Incorporating BOF slag as a fine aggregate replacement not only supports sustainable development goals but also reduces costs, offering a viable solution for affordable housing in infrastructure projects.