A facile strategy was put forward to fabricate highly luminescent metal–organic frameworks (MOFs) with tunable and white-light emission by incorporating lanthanide cations (Ln³⁺) into the channels of MIL-121 (Al(OH)(H₂btec)·H₂O) whose uncoordinated carboxyl groups can act as post-synthetic modification sites. The intense luminescence of Ln³⁺ doped MIL-121 indicates that the framework with rigid, permanently porous structure and non-coordinated carboxyl can serve as both a scaffold and an antenna for hosting and sensitizing the luminescence of Ln³⁺ cations. Moreover, a fine-tuning of the emitted color luminescence can be easily achieved by simply modulating the doping ratio or adjusting the excitation wavelength. Notably, the red–green–blue-based white light emitting Ln-MOFs can be realized by simultaneously doping Eu³⁺ and Tb³⁺ into the host framework. In addition, because of the excellent luminescence and the structural stability of Ln³⁺ functionalized MIL-121 (Ln³⁺@MIL-121) in water or solvents, the Eu³⁺@MIL-121 was developed as a luminescent probe for sensing of anions in aqueous solutions and small organic molecules. Luminescent studies reveal that Eu³⁺@MIL-121 not only display a high-sensitivity sensing function with respect to fluoride and dichromate ions but also exhibit significant solvent-dependent luminescent response to small-molecule pollutants, such as chloroform and acetone.