π-Electron magnetic compounds on surfaces have emerged as a powerful platform to interrogate spin interactions at the atomic scale, with great potential in spintronics and quantum technologies. A key challenge is organizing these compounds over large length scales, while elucidating their resulting magnetic properties. Herein, we offer a relevant contribution toward this objective, which consists of using on-surface synthesis coupled with coordination chemistry to promote the self-assembly of π-electron magnetic porphyrin species. A porphyrin precursor equipped with carbonitrile moieties in a trans arrangement was prepared by solution synthesis and deposited on Au(111)/mica. Depending on the specific growth protocol, surface-promoted reactions led to the transformation of the precursor into non-magnetic Au-CN coordinated porphyrin monomers, covalent porphyrin dimers, and one-dimensional porphyrin polymers (based on porphyrin monomers or covalent porphyrin dimers), as revealed by scanning probe microscopy studies combined with theoretical calculations. Interestingly, the scanning tunneling microscopy tip could convert such closed-shell porphyrin units into open-shell species by the removal of some peripheral hydrogen atoms. The magnetic features (i.e., singlet or triplet ground state) of the porphyrin units comprising the polymers were investigated for polymers of different lengths. No magnetic exchange coupling between adjacent units was observed, suggesting protection of the magnetic entities.