Quasi-2D halide perovskites have recently emerged as a promising family of semiconducting materials for light-emitting applications. However, these materials usually consist of mixed 2D-3D perovskite phases with rarely controlled distributions when processed into thin films. Such film structures can be detrimental to carrier transport and transfer, limiting the electroluminescent properties and light-emitting device performance. In this work, we tailor the solution crystallization of quasi-2D perovskites with a nanocrystals-mediation strategy, which leads to high-quality, low-defect-density quasi-2D perovskite films with uniformly mixed 2D-3D perovskite phases. Mechanistic insights gained from ultrafast spectroscopic studies reveal a highly enhanced carrier funnelling process in such nanocrystals-mediated films, which results in a threefold increase in the electroluminescence efficiency compared with the pristine thin pristine films. This work demonstrates the promise of a potentially versatile approach in using nanocrystals to manipulate the functional properties of quasi-2D perovskite thin films.