The paper presents the results of systematically characterizing the bottom and fly ash from a typical water-cooled vibrating grate furnace and the fly ash from a circulating fluidized-bed (CFB) combustor of two power plants burning similar mixed agricultural residues. Multi-standard techniques were employed in the characterization in terms of basic properties, particle morphology, chemical composition, mineralogy, and leaching behaviors. It was found that, despite the similar elemental compositions, the basic properties, particle morphology, mineralogical composition, and leaching behaviors were quite different between the three ashes mainly because of the differences in combustion conditions and, consequently, ash formation behaviors. The characterization indicated that the grate furnace bottom ash and CFB fly ash have potential for agricultural use with proper handling, while the grate furnace fly ash is currently problematic for use on agricultural purposes as well as building material. The obtained data and implications of this study are valuable for evaluating ash quality and developing sustainable utilization and management of the ashes from power plants firing mixed agricultural biomass.