AbstractIn natural systems, temperature‐induced assembly of biomolecules can lead to the formation of distinct assembly states, created out of the same set of starting compounds, based on the heating trajectory followed. Until now it has been difficult to achieve similar behavior in synthetic polymer mixtures. Here, a novel pathway‐dependent assembly based on stimulus‐responsive polymers is shown. When a mixture of mono‐ and diblock copolymers, based on elastin‐like polypeptides, is heated with a critical heating rate co‐assembled particles are created that are monodisperse, stable, and have tunable hydrodynamic radii between 20 and 120 nm. Below this critical heating rate, the constituents separately form polymer assemblies. This process is kinetically driven and reversible in thermodynamically closed systems. Using the co‐assembly pathway, fluorescent proteins and bioluminescent enzymes are encapsulated with high efficiency. Encapsulated cargo shows unperturbed function even after delivery into cells. The pathway‐dependent co‐assembly of elastin‐like polypeptides is not only of fundamental interest from a materials science perspective, allowing the formation of multiple distinct assemblies from the same starting compounds, which can be interconverted by going back to the molecularly dissolved states. It also enables a versatile way for constructing highly effective vehicles for the cellular delivery of biomolecular cargo.