CaO- and MgO-rich fly ash derived from low-grade coals is typically discarded due to its incompatibility with existing large-scale valorisation technologies. This study demonstrates the potential of such low-grade fly ash as a substitute for flux and MgO sources in the iron-ore sintering process, a critical step in producing feedstock to yield ∼1 Gt/yr of crude steel globally. In situ X-ray diffraction (XRD) analysis confirmed the supply of fine MgO particles from fly ash via thermal decomposition. Aligned with other MgO sources, fly ash suppressed the formation of calcium ferrites while increasing spinel content. Notably, the addition of fly ash enhanced the physical strength of sinter tablets due to the high reactivity of the MgO particles, which is in marked contrast to the detrimental effect of conventional MgO sources. Removal of sulphur from the fly ash further augmented the physical strength of the tablets by enhancing the reactivity of MgO. Further, detrimental elements for sintering, i.e., Na, K, P, and Cl, were negligible in the fly ash. These findings highlight the potential role of fly ash in supplying MgO without compromising the physical strength of the sinter, offering insights into sustainable steel production.