Constructed wetlands are one of the most appropriate wastewater treatment systems in mountain areas, where altitude, slope and climate constitute major environmental and economic constraints for infrastructure construction and subsequent management. In order to protect mountain natural wetland habitats that are sensitive to ecological equilibrium disruption, instead of the more commonly used macrophytes, plant species native to upland wetlands should be preferentially implemented as a contribution to biodiversity conservation and for the creation of more efficient, more resilient and better-adapted constructed wetlands. Carex paniculata is a key macrophyte in several European mountain aquatic habitats, and one of the few high-biomass producers that can grow at sea level and at altitudes of up to 2,600 m. In this paper, the results of a 2-year investigation demonstrate the efficacy of Carex paniculata for the treatment of the mixed stormwater, sewage and livestock wastewater effluents from a typical rural settlement at 825 m above sea level in the Cantabrian Mountains. The year-round suitability of Carex paniculata for the treatment of wastewater with seasonally variable flow and composition in mountain areas is demonstrated.