The harsh radiation environment on Mars is widely believed to destroy organic matter, but elucidating the systematic degradation pathway and its controlling factors has proved elusive. Here we show the alteration of macromolecular organic matter in the ∼500-m.y.-old uranium-rich Alum Shale Formation (northwestern Europe), which is suggested as a geological analogue on Earth of the Martian surface samples, in response to ⟨-particle irradiation. The composition and quantity of pyrolysates as well as the extractability of immature Alum Shale are governed by uranium content, and this has been confirmed independently using uranium-rich shales of younger geological age. During the course of irradiation, macromolecules undergo cross-linking, demethylation, and aromatization processes, which are active at different stages in the alteration of the organic matter. A comparison of the Martian surface samples with Kolm (carbonaceous nodules with very high uranium content intercalated in the Alum Shale) samples in terms of irradiation dosage and organic geochemical characteristics reveals that they have received similar levels of irradiation. A model of the radiation-induced alteration of organic matter has been developed, which opens a window for understanding the possible pristine nature and composition of the irradiated organic matter on Mars and offers arguments to qualitatively predict potential organic matter in the Martian paleo-lacustrine subsurface.