With the increasing demand for energy and growing environmental concerns, sodium-ion batteries have garnered significant attention in recent years as a potential alternative to lithium-ion batteries. This is due to their low cost, environmental friendliness and abundant raw materials. In particular, manganese-based cathode materials have gained popularity for their high capacities and abundant precursor materials. However, poor air stability of these materials is a major hindrance to their commercialization. Due to the nature of the layered structure with large Na-ions, both carbon dioxide and moisture can cause cathode materials to deteriorate even after a few hours of exposure to air, which leads to a loss of structural and chemical integrity. The air stability of cathode materials is essential for manufacture, storage, performance, and long-term cycling of batteries. Several factors and mechanisms are reported to influence the air stability of these materials, such as the composition, the presence of dopants, the particle size, and the method of preparation. This review paper comprehensively investigates the causes, measurements, mechanisms, solutions, and future perspectives of air stability of cathode materials for sodium-ion batteries.