Abstract: Based on the "antenna effect", organic ligands efficiently absorb ultraviolet light and transfer the harvested energy to lanthanide ions via triplet states, endowing lanthanide metal–organic frameworks (Ln-MOFs) with intense characteristic fluorescence. This review focuses on antenna effect-based sensing mechanisms in Ln-MOFs. This process facilitates efficient energy transfer from light-harvesting organic ligands to lanthanide centers, thereby overcoming the weak direct excitation of Ln³⁺ ions. Fluorescence quenching mechanisms include competitive absorption of excitation energy between the analyte and the Ln-MOFs, disruption of ligand-to-metal energy transfer caused by analyte–framework interactions, and structural collapse of the MOFs upon analyte binding. In contrast, fluorescence enhancement mechanisms involve displacement of coordinated water molecules from the Ln³⁺ sphere, increased framework rigidity that suppresses non-radiative decay, and analyte-mediated sensitization that creates new energy transfer pathways. Notably, ratiometric dual-emission probes, particularly mixed Eu³⁺/Tb³⁺ systems, generate visually distinguishable color gradients under ultraviolet light, enabling semiquantitative naked-eye detection. These systems represent a promising strategy for on-site rapid sensing without sophisticated instrumentation. On this basis, the review systematically summarizes recent advances in the application of Ln-MOFs for detecting metal ions (e.g., Hg²⁺, Pb²⁺, Fe³⁺, Al³⁺), anions (e.g., Cr₂O₇ ²⁻, PO₄ ³⁻, MnO⁻ ₄, F⁻), and small organic pollutants (e.g., antibiotics, nitroaromatic compounds, and biomarkers) in aqueous media. Despite their high sensitivity and selectivity in laboratory studies using pure water or buffer solutions, systematic evaluations under real water matrix conditions remain limited. Key challenges include pH fluctuations, competing ions, and interference from natural organic matter, all of which can significantly affect Ln-MOFs fluorescence responses. Moreover, although Ln-MOFs generally exhibit better stability in water than transition-metal MOFs due to the high charge density of Ln³⁺ ions, their long-term stability has not been systematically benchmarked across different material systems. Furthermore, comprehensive toxicity assessments remain lacking. The potential environmental and biological risks associated with lanthanide ions, organic ligands, and residual synthesis solvents are still largely unexplored. A limited number of studies have demonstrated the feasibility of Ln-MOFs in real water samples, such as the visual detection of fluoride ions in river water using boronic acid-functionalized mixed Ln-MOFs and the ratiometric detection of Al³⁺/F⁻ in tap and river water. However, the anti-interference capability and long-term stability under continuous exposure to real water matrices require further in-depth investigation. Future research should prioritize (i) spiked recovery and anti-interference tests in diverse real water samples, (ii) development of portable detection platforms based on smartphone-assisted readout or test strip colorimetric analysis, and (iii) systematic evaluation of the environmental fate and biocompatibility of Ln-MOFs to accelerate their practical deployment in water quality monitoring.