Northeast China possess large areas of agricultural farmlands that were reclaimed from earlier marsh wetlands in the past several decades, while the response of trace element like mercury (Hg) in this process were not fully understood. In the present study, total mercury (THg) and Hg isotope composition were determined in reservoir sediment cores of a reservoir, agricultural soils and forest soils in Northeast China to reveal the possible impacts of agricultural activities on Hg geochemistry in the reservoir. The more negative δ²⁰²Hg (↓0.35‰), increased concentrations of THg (↑22.4 ng g⁻¹), and higher organic matter (OM) (↑10%) in the deeper than the upper layer sediments indicate noticeable Hg input about 50–60 years ago, which is coincident with the grand reclamation of marsh wetland in the study area. The gradual decrease in THg content and the increase of δ²⁰²Hg value in the upper sediment layer reflect the weakened reclamation and thus less Hg input in the later decades. Using a triple mixing model, it is estimated that more than half of Hg in this reservoir sediment was originated from the reclamation of marsh wetlands into farmlands in the earlier stage and soil erosion in the later period. The study demonstrated that Hg isotope signatures in reservoir sediment core can reflect the landscape changes, such as reclamation.