Metal (loid) soil pollution is an important issue worldwide that requires the use of remediation, using plants for instance. However, for plants to grow and be efficient, soil fertility must be improved and metal(loid) toxicity reduced. In such an assisted remediation context, biochar has been shown to improve soil conditions and thus plant growth. However, few field studies over long time periods have been conducted to assess biochar impact on metal(loid)s in soil. The objectives of this study were to evaluate the efficiency of a biochar amendment, under field conditions, to improve soil physico-chemical properties and immobilize metal(loid)s and to study the possible mechanisms of such metal(loid) immobilization. For this, biochar was applied to a former mine technosol highly contaminated with As and Pb. Soil samples were taken prior to biochar amendment, on the day of biochar application and 6 months after biochar incorporation. The analyses showed that biochar increased soil pH, organic matter content and nutrient availability, and immobilized Pb. Such results were observed directly after biochar application and were still observed after 6 months, whereas As was first mobilized then immobilized. Finally, the analysis of the samples using scanning electron microscopy coupled with energy dispersive spectroscopy showed that Pb was immobilized, at least partly on the biochar surface, as PbCO3.