Steel-making slag is largely used today in road construction and other applications, but significant volumes are landfilled and cannot be recycled for excessive contents in hazardous metals, such as chromium or vanadium. The long-term behaviour of this material is still little known, and the characterisation of large volume slag dumps remains an environmental challenge. In this study various analytical techniques are used to characterise Electric Arc Furnace (EAF) slag landfilled for several decades and exposed to chemical weathering and erosion. Coupling chemical, mineralogical and magnetic techniques helps to understand the relations between hazardous metals and mineral phases. A special interest is given to Fe-bearing minerals microstructure so as to link the magnetic properties of the material to its mineralogical composition. The studied slag presents high amounts of chromium (between 1 and 3 wt. %) and very high magnetic susceptibility values (near 60 × 10−6 m3/kg), explained by the presence of magnetite and a spinel solid solution. Some correlations are found between magnetic susceptibility and potentially hazardous metals, providing new perspectives for future environmental investigations.