Delocalized p-shell electron magnetism emerging in a low-dimensional graphene system due to quantum effect is distinct from the localized d/f-shell electron’s. The delocalization effect allows the precise engineering of the magnetic ground state and magnetic exchange interactions in nanographenes, thus implementing the accurate construction of high-quality graphene-based magnetic quantum materials. In recent years, with the development of surface chemistry and surface physics, it has become feasible to study the magnetism of nanographenes with single-atom precision, thus opening a new research direction for studying purely organic quantum magnetism. This review starts from the summarizing of the research background of nanographene magnetism. Then, the physics nature behind the nanographene magnetism and recent experimental researches are discussed. Finally, the challenges and opportunities for further studying low-dimensional magnetic graphenes are briefly discussed.