The steel industry produces significant quantities of by-products, notably steelmaking slags, which are often underutilized and disposed of through environmentally detrimental means such as landfilling. This study explores the valorization of steelmaking slags—particularly from basic oxygen furnace (BOF) and electric arc furnace (EAF) processes—for sustainable applications in biomass conversion and biomanufacturing. The chemical composition and physicochemical properties of these slags make them suitable as catalysts, nutrient sources, and adsorbents in a variety of biomass transformation pathways. Through thermochemical and biochemical processes, slags can enhance biofuel and biochemical production, contribute to carbon sequestration, and support microbial processes. Key applications include catalyzing biomass pyrolysis, enhancing biochar production, and supporting fermentation through micronutrient supply. Life cycle assessments reveal that integrating slag valorization reduces greenhouse gas emissions, supports circular economy initiatives, and improves the economic feasibility of biomanufacturing processes. Despite technical challenges such as material heterogeneity and potential leaching risks, steelmaking slag utilization demonstrates considerable promise in transitioning heavy industries toward sustainability. Successful case studies show potential for industrial symbiosis, combining steel production with bioenergy and bioproduct development. The paper concludes with policy recommendations and research directions to further optimize slag valorization and integrate it into large-scale biomanufacturing systems.