Porphyrin-based metal coordination polymers (MCPs) have attracted significant attention due to their great promise for applications in phototherapy, including photodynamic therapy (PDT) and photothermal therapy (PTT). However, the detailed self-assembly process of porphyrin-based MCPs is still poorly understood. This work provides a detailed study of the self-assembly process of MCPs constructed from Mn2+ and TCPP (TCPP: 5,10,15,20-tetrakis(4'-carboxyphenyl)porphyrin) in aqueous solution. Unlike the traditional nucleation and growth mechanism, we discover that there is a metastable metal-organic intermediate which is kinetically favored in the self-assembly process. And the metastable metal-organic intermediate nanotape structures could convert into thermodynamically favored nanosheets through disassembling into monomers followed by a reassembling process. Moreover, the two structurally different assemblies exhibit distinct photophysical performances. The intermediate Mn-TCPP aggregates show good light-induced singlet oxygen 1O2 generation for PDT while the thermodynamically favored stable Mn-TCPP aggregates exhibit an excellent photothermal conversion ability as photothermal agents (PTAs). This study could facilitate the control of the self-assembly pathway to fabricate complex MCPs with desirable applications.