Despite the enormous advancements in nanomedicine research, a limited number of nano formulations are available on the market and translated to clinics. Easily scalable, sustainable, cost-effective manufacturing strategy and long-term stability for storage are crucial for successful translation. Here, we report a system and method to instantly formulate NF achieved with a nanoscale polyelectrolyte coacervate-like system, consisting of anionic pseudo-peptide poly (L-lysine iso-phthalamide) derivatives, polyethylenimine, and doxorubicin (Dox) via simple ‘mix-and-go’ addition of precursor solutions in seconds. The coacervate-like nanosystem shows enhanced intracellular delivery of Dox to patient-derived multidrug-resistant (MDR) cells in 3D tumor spheroids. The results demonstrate the feasibility of an instant drug formulation using a coacervate-like nanosystem. We envisage that this technique can be widely utilised in the nanomedicine field to bypass the special requirement of large-scale production and elongated shelf-life of nanomaterials.