A facile, solid-state mechanochemical method was developed for the synthesis of low molecular weight chitosan (LMWC) in one step from chitin and crude shrimp shell powders, in which chitin undergoes simultaneous deacetylation and depolymerisation in the presence of a base catalyst under mechanical milling. The method is advantageous over traditional multi-step methods, featuring enhanced efficiency and significantly reduced environmental impact. Compared to traditional approaches, the base usage is reduced to about 1/10, and the molecular weight of the obtained LMWC product is much more narrowly distributed, with a polydispersity value of only 1.1. The degree of deacetylation and the molecular weight can be adjusted by varying the ball milling parameters. The influences of different types of bases were investigated by solid-state NMR analysis and control experiments, pointing out the critical role of base in both depolymerisation and deacetylation. Base-catalysed mechanochemical transformation of chitin and shrimp shells provides a solvent-free way to effectively valorize shellfishery waste for valuable products.