The present research reports the chemical and mineralogical results of various archaeometallurgical lead slags from the Ari site at Lavrion, one of the largest ancient sites of Pb–Ag metallurgical activities in Greece, excavated by the School of Mining and Metallurgical Engineering of Greece, under the directorship of Prof. C. Tsaimou. Dating of the metallurgical slags was carried out using radiocarbon measurements, indicating the earliest phase of the Roman Empire. Chemical analysis, X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR) and scanning electron microscopy (SEM) were used to identify the Ca-rich silicate, oxides and metallic phases. The major constituents of the slag proved to be melilite (akermanite/hardystonite), clinopyroxene, olivine, spinel and glass. Zinc was dissolved in the silicate melt and was mostly present in the form of the zinc member of the spinel and as silicate (hardystonite). Lead, in contrast, behaves as an “incompatible element” and was likely to be concentrated in the residual matrix. Pb was also present in the form of metallic inclusions.