Earth’s marine nitrogen cycle has co-evolved with life and redox conditions over geological time. In this Review, we provide an account of nitrogen cycling in the world’s oceans over the past ~4 Ga, from the dawn of life to the modern day. Stable nitrogen isotopes from sedimentary rocks, paired with other proxies, provide evidence that the nitrogen cycle has responded to and perhaps modulated events such as the emergence of life, oxygenation events, major climatic perturbations, and mass extinction events. Before the evolution of nitrogen fixation, bioavailable nitrogen was supplied via processes such as lightning, photochemical reactions, meteorite impacts and hydrothermalism. The advent of microbial N2 fixation facilitated the expansion of ecosystems. Establishment of a marine nitrate reservoir in the Neoproterozoic (1,000–541 Ma) probably enabled eukaryotic algae to dominate ocean primary productivity. Phanerozoic nitrogen cycle transitions over 100-Myr timescales are associated with icehouse-to-greenhouse conditions. Short-lived perturbations occurred during mass extinctions and anoxic events, which are linked to evolutionary changes, climatic extremes and ocean stagnation. The impact of the terrestrial biosphere on the global marine nitrogen cycle remains poorly resolved and should be addressed in future research to help answer open questions about the spatial and temporal trends in nutrient availability over Earth’s history.