The present manuscript probes the decomposition behaviour of the ferromagnetic binary metastable Heusler alloy in the Al-Mn system. The ferromagnetic ordered tetragonal phase (L1(0), popularly known as tau phase), which is metastable in nature, was synthesised from the melt through a copper mould chill casting process using 10-mm diameter mould. Isothermal and non-isothermal heat treatments were carried out to understand the decomposition sequence of tau phase at high temperatures. The metastable phase undergoes, on heating, a eutectoid-like decomposition of , where is the intermetallic phase Al8Mn5. Both the phases are present in the equilibrium diagram. The two phases at higher temperature further transform to a single phase epsilon having a hexagonal structure. We show through in situ X-ray diffraction studies that transforms to a disordered bcc phase () prior to the transformation to the epsilon-phase. The overall sequence of phase decomposition during heating can be given as . The high-temperature phase cannot be retained through quenching experiment and its existence can only be established by in situ studies.