Mercury is a highly toxic pollutant and persistent in the sediment, which highlights the needs for remediation of sediment Hg pollution. However, the effects of remedial actions on the pollution and ecological risk of Hg in sediment are less investigated. Therefore, this study conducted an environmental risk assessment before and after corrective actions in the brine water storage pond of a closed alkali-chlorine plant with high Hg pollution in the sediment. The results showed that the accumulation of Hg in the sediment (2.59–443 mg/kg), fish and crabs (1.10–8.54 mg/kg) in the polluted pond was higher than the regulation limit in Taiwan. After implementing the corrective actions such as institutional/engineering control and remediation, we found that the Hg concentration and pollution factor (CF), in the sediment were significantly decreased by 74% and 73% (p = 0.02), respectively. In addition, the geoaccumulation index (I-geo) were decreased to lower pollution class after corrective actions (p = 0.009). The risk related indices such as potential ecological risk index (RI) and risk quotient (RQ) also showed significant decreases (p = 0.03) after corrective actions (71% and 73%, respectively). Although the values of pollution and risk indices were still high after remediation, the results of this study demonstrated the effectiveness of corrective actions on amelioration of sediment Hg pollution. It suggests that corrective actions should be continuously implemented to reduce the pollution and risk levels in all aspects to an acceptable level for stakeholders.