Abstract: The process of separation holds immense significance in chemical production, the national economy, and the construction of national defense. Membrane separation technology plays a pivotal role due to its high efficiency, energy conservation, and eco-friendliness. Mixed-matrix membranes (MMMs) consist of a dispersed nanomaterial phase and a continuous polymer matrix, hoping to combine the high intrinsic permeability and separation properties of molecular sieves with the robust processing and mechanical properties of polymers. This review primarily outlines the advancements and groundbreaking efforts in MMM development. With a diverse range of filler options, MMMs demonstrate superior performance across various separation scenarios. Notably, a team from Nanjing Tech University has proposed a solid solvent processing technique for fabricating these membranes. In contrast to traditional solution mixing methods, this approach uses the polymer as a solid solvent for dissolving metal salts, resulting in an ultrathin metal salt@polymer precursor layer. The metal salt is immobilized by the polymer and undergoes in situ conversion to a metal-organic framework (MOF) through ligand vapor treatment. The solid solvent maintains the MMM integrity and inhibits the agglomeration of MOF particles. Additionally, the flexible polymer segment tightly attaches to the generated MOF particles, resulting in an intact MOF-polymer interface. This method allows for precise control of membrane thickness, reducing it to below 100 nm, while increasing doping levels by 2—4 times. The membrane performance is an order of magnitude enhancement in membrane permeability and selectivity. This innovative approach establishes a novel MMM structure where the filler takes precedence, complemented by the polymer. This not only maintains excellent processability and the potential for scale-up production, but also elevates the separation performance of mixed-matrix membranes to a level akin to inorganic molecular sieve membranes.