Directed evolution (DE) is a process of mutation and artificial selection to breed biomolecules with new or improved activity. DE platforms are primarily prokaryotic or yeast-based, and stable mutagenic mammalian systems have been challenging to establish and apply. To this end, we developed PROTein Evolution Using Selection (PROTEUS), a new platform that uses chimeric virus-like vesicles (VLVs) to enable extended mammalian DE campaigns of RNA-encoded transgenes without loss of system integrity. This platform is stable and can generate sufficient diversity for DE in mammalian systems. Using PROTEUS, we altered the doxycycline responsiveness of tetracycline-controlled transactivators, generating a more sensitive TetON-4G tool for gene regulation. PROTEUS is also compatible with intracellular nanobody evolution, and we use it to design a DNA damage-responsive anti-p53 nanobody. Overall, PROTEUS is an efficient and stable platform to direct evolution of biomolecules within mammalian cells, and represents a valuable tool for engineering improved mRNA therapeutics for human health.