Nanostructured graphitic‐layer‐materials with precise control of the layer‐nanostructure, are known to surpass many benchmarks in electrical, optical, and mechanical properties. The development of such controlled synthesis is, however, stalled by the difficulty in tracking the exact growth mechanism and dynamics of the layer‐structure. Herein, the growth mechanism of onion‐like graphitic‐layer‐structures with an atomic precision is revealed by pyrolysis in an aberration‐corrected environmental transmission electron microscopy (ETEM). Specifically, the time‐evolution of cobalt phthalocyanine (CoPc), bearing better contact between carbon atoms and metamorphosizing a graphitization‐catalyst, at 850°C in an ETEM are tracked to an intriguing Co‐Co3C nanocore enveloped by several graphitic layers. The growth dynamics of this onion‐like graphitic shell comprises, rather unexpectedly, out‐diffusion of carbon atoms from the core to fuel the growth of new outermost shell‐layers, plus lateral/inwards and intrashell/intershell diffusion of carbon atoms to amend shell‐defects. Thus, unusual dynamics of seemingly contracting shell‐expansion and shell‐consolidation is revealed, with the surprising phenomenon of a decrease in the number of atomic shell‐layers in exchange for layer‐perfectness towards the end of the controlled synthesis. These results indicate pyrolysis of an organometallic compound in an ETEM is a paradigm for understanding and developing controlled synthesis of novel high‐quality graphitic‐layer‐materials.