Fly ash is considered as an under-utilized product of pulp and paper industry and is mainly land-filled. However, it can be repurposed as an adsorbent for organics of wastewater effluents. Despite efficient adsorption capability, its metal components may dissolve in wastewater and harm the environment. This investigation focused on the leaching behavior of metals from biomass-based fly ash in water at pH 6 and 12.5. A similar investigation was performed in the spent liquor of a pulping process to evaluate the extraction of metals from fly ash in such an environment that fly ash could be used as an adsorbent. The results revealed that the predominant metals leached from fly ash in water and the spent liquor were Ca, K, Mg, Mn, Na, and Si. The trace metals including Al, Ba, Sr, and Zn were also detected to a significant extent. Interestingly, the extraction of metals from fly ash in spent liquor was more limited than in water, which is beneficial for the application of fly ash in spent liquors.