The demand for thermally stable aluminum alloys is increasing, but mechanical strength of most of existing alloys inevitably decreases when exposed to high temperature environments. In this study, newly developed powder metallurgy (P/M) Al–Mn alloy extrusions were subjected to full annealing treatment and temperature-accelerated test, to simulate decrease in hardness after prolonged service periods at high temperatures, and were found to possess higher thermal stability than a conventional ingot metallurgy (I/M) 3000 series Al–Mn alloy extrusion. Such improvement of high temperature strength was attributed to the increased volume fractions of thermally stable Al6Mn phase, and quantitatively explained according to the underlying strengthening mechanisms, by which their terminal strength after prolonged and thus practically unmeasurable periods could be predicted. Prediction of the microstructures and the corresponding Vickers hardness was also achieved by combining phase-field simulation and classical micromechanics approach, confirming that the utilized simulation model could enable to predict the terminal strength of the developed P/M alloy extrusions.