Directing self-assembly processes out-of-equilibrium to yield kinetically trapped materials with well-defined dimensions remains a considerable challenge. Kinetically controlled assembly of self-synthesizing peptide-functionalized macrocycles through a nucleation-growth mechanism is reported. Spontaneous fiber formation in this system is effectively shut down as most of the material is diverted into metastable non-assembling trimeric and tetrameric macrocycles. However, upon adding seeds to this mixture, well-defined fibers with controllable lengths and narrow polydispersities are obtained. This seeded growth strategy also allows access to supramolecular triblock copolymers. The resulting noncovalent assemblies can be further stabilized through covalent capture. Taken together, these results show that self-synthesizing materials, through their interplay between dynamic covalent bonds and noncovalent interactions, are uniquely suited for out-of-equilibrium self-assembly.