Heat treatment of additively manufactured Al-Ce based multicomponent alloys leads to complex microstructure evolution. In this research, the ability to extend the phase transformation theories involving nucleation of a product phase from a heterogeneous multi-phase microstructure typical to that of additively manufactured samples is explored. The Al-10Ce-8Mn (wt%) was used as a model alloy system. Under additive manufacturing conditions different solidification microstructures were obtained due to spatial and temporal variations of thermal gradients (G) and liquid-solid interface velocities (R) within a given melt pool. Near the melt pool boundary (high G and low R, referred as MPB region), initially, Al20Mn2Ce forms from the liquid followed by a eutectic of FCC Al and Al11Ce3. In the melt pool interiors (low G and high R referred as ES region) a eutectic structure between FCC Al and Al20Mn2Ce is observed. During subsequent heat treatments, the MPB and ES regions transform into different sets of microstructures. In the MPB region, a fine globular microstructure containing FCC Al, Al11Ce3, Al6Mn, and Al12Mn results from the decomposition of Al20Mn2Ce. In the ES region a faceted Al51Mn7Ce4 plate phase results from the decomposition of Al20Mn2Ce. The formation of the Al51Mn7Ce4 phase within the eutectic microstructure at the boundaries of FCC Al and Al20Mn2Ce has not been reported in the literature. These two distinct phase transformation pathways are rationalized based on the role of driving force on the nucleation of (Al6Mn) and/or metastable intermetallic (Al51Mn7Ce4) phases at the interface of aluminum (FCC) and the non-equilibrium intermetallic (Al20Mn2Ce) phases.