Carbon nano-onions are unique, tiny, multilayered carbonaceous nanoparticles having distinct structural, microstructural, and a range of physical characteristics and high-tech uses. Similar to other carbonaceous nanoparticles (graphene, carbon nanotubes), carbon nano-onions, as well as modified carbon nano-onions, have been considered as valuable nanofillers for macromolecules. Along these lines, the present overview refers to the scientific impact of reinforcing carbon nano-onions in polymeric matrices. For this purpose, this manuscript has been systematically categorized into an initial section addressing fundamentals of carbon nano-onions, followed by a major section regarding design, structure-property, and performance aspects of carbon nano-onion nano-reinforced polymeric matrices (conducting, thermoplastics, thermosets). Subsequent review sections unfold promising application zones, challenges, and the future of polymer/carbon nano-onions hybrids. According to the literature, numerous facile and efficient routes (in situ, electrochemical, solution, solvent casting, curing) have been used for fabricating these nanomaterials. Herein, adding carbon nano-onions was noticed to valuably enhance the morphological, physical, and applied scope of the ensuing nanocomposites. Owing to design diversities, a myriad of technological deployments have been noticed for polymer/carbon nano-onions, including space engineering-to-energy production/storage/sensing devices and catalysis. Nevertheless, delimited systematic scientific studies have been performed so far regarding different types of polymer/carbon nano-onions and their applied specifications. Henceforth, focused and continuous research efforts by the field scientists/engineers seemed to be indispensable for overcoming the underlying challenges to attain next-generation industrial-scale carbon nano-onions derivative nanomaterials.