An accurate characterization methodology is indispensable to design nanoparticle‐based drug delivery system (DDS) adapted to specific diseases and therapies. However, characterization techniques employed to investigate drug release and nanoparticle degradation require separating the nanoparticles from their suspension media, which can lead to artefacts. Therefore, there is a clear need to implement novel versatile in‐situ methods. Here, we report the use of in‐situ NMR spectroscopy to monitor drug delivery processes from MOF nanocarriers, both in solution and in the solid state simultaneously. In‐situ1H NMR investigation of nanoMIL‐100(Al) suspension in phosphate medium enabled recording the trimesate ligand loss in the solid phase and the ligand release in the liquid phase as a function of time. Simultaneously, 27Al NMR enabled assessing the progressive replacement of carboxylate ligands by phosphates leading to the formation of new aluminum species. Using the same strategy, we also compared the degradation of nanoMIL‐100(Al) loaded with two drug analogs, highlighting an effect of metal‐ligand complexing strength. Furthermore, our in‐situ technique is applicable to studying the reaction of paramagnetic nanoMIL‐100(Fe) in the liquid phase. This work offers an alternative to ex‐situ techniques for understanding the degradation mechanism of MOF nanocarriers and could be an asset for other nanoscale DDSs.