Pristine peatlands are typically low in nitrogen, sulfur and metal compounds. Thus, input of high concentrations of those compounds as a result of anthropogenic activity pose a huge challenge to peatland ecosystems. At a mine site in Finnish Lapland, mining-affected waters are purified in two treatment peatlands (TPs) before they are released into downstream waters. The TPs experience long winters and are snow- and partly ice-covered from October to May. Contaminants in inflow waters include nitrogen compounds, sulfate, metals and metalloids. The TPs were intensively monitored for >10 years, and monitoring data was complemented with laboratory experiments. High levels of multiple contaminants, often in the mM range, were measured in TP inflow. Removal of some contaminants such as nitrogen compounds and sulfate was higher in summer months while removal of other contaminants such as arsenic and antimony was similar throughout the year. Potential process rates as assessed in laboratory incubations were generally higher at higher incubation temperatures and decreased with decreasing temperatures, but processes still occurred at 0 °C. The composition of the potentially active microbial community as assessed by 16S rRNA amplicon sequencing varied more strongly between the two TPs and the two layers, while seasonal variability was minor. Potentially active microorganisms included genera known for nitrification, denitrification, sulfate reduction, iron reduction as well as arsenate and antimonate reduction. The collective results indicate that (i) microbial communities in mining-affected peatlands were exposed to high concentrations of multiple contaminants, (ii) microbially mediated processes contributed to contaminant removal throughout the year, and (iii) differences in process rates and contaminant removal likely stem from overall lower activities rather than from changes in microbial community composition.