The environmental challenges have been a long-standing concern and have caused severe damages up until now. Wastewater management and the emission of greenhouse gases are among these challenges. Impure and contaminated water can cause danger to both human lives and the ecosystem. Greenhouse gas emission has also been a crucial reason for changes in atmospheric dynamics and the increase of pollution in the environment. The rate of population increase has caused an increasing demand for fresh air and fresh water. Polymer nanocomposite membranes have been developed to overcome these challenges and facilitate the separation process for wastewater treatment and gas separation. Restrictions for polymeric membranes, such as compromise among water permeability, solute selectivity, and low resistance to fouling, have led to development of next-generation membranes that can possess these properties altogether. These nanocomposite membranes are incorporated with nanofillers, which introduce exceptional abilities to the membranes. This chapter presents a comprehensive analysis of the latest advancements in water treatment and gas separation for environmental applications, highlighting the recent advancements in membrane manufacturing technology. In this chapter, the focus is on how nanofillers, such as carbon nanotubes (CNTs), graphene oxide (GO), and titanium dioxide (TiO2), impact the performance of membranes and improve their properties. This area is also examined regarding its challenges and opportunities, along with an investigation into potential future directions for using these materials in environmental applications.