In recent decades, the world has experienced a surge in extreme environmental events, exacerbated by the ongoing impacts of climate change. Substantial research underscores the role of anthropogenic CO2 emissions as the primary driver behind climate change and global warming. Cement production, constituting roughly 7% of global anthropogenic CO2 emissions, stands as a significant contributor to this issue. Consequently, there has been a growing focus on mitigation strategies within the cement industry, particularly emphasizing the utilization of mineral admixtures. An innovative yet underexplored material for partial cement replacement is iron mining waste rock, a by-product of open-pit mining operations used to extract iron ore deposits. Unlike underground mining, open-pit methods generate larger volumes of waste rock, typically stored in substantial geotechnical structures like piles, pits, or dams. This study conducted physical–chemical, mineralogical, and morphological characterizations on iron mine waste rock. Subsequently, four mortar mixtures were formulated, integrating iron mine waste as a partial substitute for cement (up to 30% by mass), and compared against a reference mortar. The performance assessment encompassed evaluations of fresh mortar properties, compressive strength, and elastic modulus. Findings revealed that the waste rock demonstrates limited pozzolanic activity, primarily functioning as a filler, and its inclusion in mortar enhances the workability of the mixes. However, a marginal reduction in compressive strength (up to 15%) was observed in mortar mixtures where 30% of the cement was replaced by waste rock.