Tremendous advances in nanoscience have been made since the discovery of the fullerenes; however, the formation of these carbon-caged nanomaterials still remains a mystery. Here we reveal that fullerenes self-assemble through a closed network growth mechanism by incorporation of atomic carbon and C(2). The growth processes have been elucidated through experiments that probe direct growth of fullerenes upon exposure to carbon vapour, analysed by state-of-the-art Fourier transform ion cyclotron resonance mass spectrometry. Our results shed new light on the fundamental processes that govern self-assembly of carbon networks, and the processes that we reveal in this study of fullerene growth are likely be involved in the formation of other carbon nanostructures from carbon vapour, such as nanotubes and graphene. Further, the results should be of importance for illuminating astrophysical processes near carbon stars or supernovae that result in C(60) formation throughout the Universe.