In recent years, green carbon nanostructure-based composites have attracted significant attention for applications in the field of environmental remediation and energy harvesting. This chapter offers an overview of sustainable carbon nanostructure-based functional composites synthesized through eco-friendly approaches. The discussion begins by introducing the fundamental concepts of carbon nanostructures, highlighting their various types and unique properties. Special emphasis is given to green synthesis strategies, which offer environmentally friendly methods for producing carbon nanostructures while minimizing toxic by-products and energy consumption. Subsequently, the chapter explores the applications of green nanostructure-based composites in areas such as environmental remediation, encompassing air filtration, water purification, and soil detoxification. Beyond environmental remediation, the chapter also discusses their applications in photovoltaic devices, piezoelectric and triboelectric nanogenerators, as well as supercapacitors and rechargeable batteries. Finally, the chapter addresses the key challenges remaining in the green synthesis, large-scale production, and practical applications of these composites. By addressing these gaps, this work aims to provide an overview of current advancements in green carbon nanostructure-based composites and their growing potential in sustainable environmental and energy technologies.