Effective treatment of toxic sludge such as nickel-containing sludge (NS) and aluminum-containing sludge (AS) and fluorine pollution is of significance but challenging due to secondary pollution and complex treatment processes. Herein, a facile waste-to-waste strategy was proposed to prepare Ni-Al layered double oxides/sludge (Ni-Al LDOs/sludge) adsorbent using NS as nickel source and AS as aluminum source via hydrothermal treatment and calcination without adding any metal salts. The Ni-Al LDOs/sludge with a large specific surface area and mesoporous structure showed high F- adsorption performance. The F- adsorption mechanism exhibited monolayer adsorption which was consistent with the Langmuir isotherm model. The kinetic results showed that fluoride adsorption onto Ni-Al LDOs/sludge follows the pseudo-second order kinetic model. Significantly, the as-prepared adsorbent possessed desirable reusability and durability. Furthermore, a possible adsorption mechanism involving electrostatic attraction and ion exchange on the surface of the Ni-Al LDHs/sludge was discussed. Therefore, this work developed a method for converting toxic sludges into functional adsorbent, which provided a new strategy for comprehensive treatment of sludge and fluorine wastewater.