2D transition metal dichalcogenides are attracting increased attention because of their excellent electronic and optical properties. Inspired by the natural weathering exfoliation of seaside rocks, a chemical weathering concept for fabricating atom-thick 2D materials from their bulk counterparts is proposed. It is experimentally demonstrated that chemical weathering-assisted exfoliation mechanism is a simple and efficient method of preparing atom-thick MoS2 and WS2 monolayers. These monolayers are difficult to prepare using other approaches. Interestingly, the as-prepared MoS2 and WS2 monolayers exhibit excellent saturable absorption and mode-locking properties in all-solid-state lasers because of intermediate states resulting from S-vacancy defects. The obtained passively Q-switched laser operation with 60 ns pulse width and ultrafast mode locking with 8.6 ps pulse width are promising for all-solid-state laser application.