Non-woody lignin can serve as a flame retardant, though its flame-retardant performance is influenced by both the source and extraction methods. However, it often falls short of industrial requirements, primarily due to insufficient uniformity and dispersibility when blended with polymers. To improve the flame-retardant performance of non-woody lignin and its polymer composites, significant research progress has been made through chemical modification, such as nitrogen modification, phosphorus modification, nitrogen-phosphorus synergistic modification, silicon modification, metal ion incorporation, and the development of nano-lignin, all of which contribute to notably improved flame retardancy. This review discusses the manufacturing process of flame retardant from non-woody lignin, the properties of resulting materials, the effects of chemical modifications, and the fundamental mechanisms of flame retardation, while also offering perspectives on its industrial development. Through a systematic review, this work aims to propose optimized strategies for developing efficient, eco-friendly, and low-cost flame retardants based on non-woody lignin.