Breiðuvík and Kækjuskörð are two neighboring extinct eruptive centers in the East Fjords of Iceland. Together, they compose the second-largest volume of silicic rock in the country (after Torfajökull, an active volcanic system in southern Iceland). We use ages and compositions of detrital zircon collected from two rivers, the Stóraá and Krossá-Kækjudalsá, to investigate the origins and longevity of silicic magmatism at Breiðuvík-Kækjuskörð. Zircon populations from the two catchments have identical median U-Pb dates (12.9 Ma), O isotopes (δ18O Vienna standard mean ocean water = 3.1‰ versus 3.3‰), and Hf isotopes (εHf = 14.7). We interpret coherence of zircon elemental and isotopic compositions to indicate that a significant volume of relatively uniform silicic material was produced in close temporal and spatial proximity between 11.2 ± 0.7 Ma and 15.0 ± 0.9 Ma (all errors are 1σ), dominated by assimilation–fractional crystallization processes. To test the robustness of this longevity estimate, we applied Monte Carlo modeling to the Breiðuvík-Kækjuskörð detrital zircon results and found the age span to be statistically resolvable at ≥2.8 m.y. While this lifespan is comparable to those of large mafic-silicic volcanic systems that have been described in other settings globally, it is the longest reported estimate for any Icelandic volcano, where typical longevity is thought to be ∼0.5–1.5 m.y. The ≥2.8 m.y. lifespan we present for Breiðuvík-Kækjuskörð is a conservative assessment, because the dates used in this study only represent the zircon-saturated period of magmatic activity. This study demonstrates that detrital zircon analysis of volcanigenic sediment provides an efficient and powerful tool that can illuminate histories of zircon-saturated magmatism at targeted volcanic centers and systems. This approach can be particularly valuable in dominantly mafic provinces where silicic material is subordinate (e.g., ocean islands, flood basalt provinces), where glaciation, erosion, or alteration has transformed the landscape, or in areas that are inaccessible (e.g., obscured by glacial ice).