We present detrital zircon U-Pb data from mainly fluvial sandstones of the Mackenzie Mountains Supergroup, Yukon (four samples), and the Shaler Supergroup, Northwest Territories (seven samples). The high similarity of data sets from widely separated units supports long-held stratigraphic correlations between the late Mesoproterozoic–Neoproterozoic Mackenzie and Amundsen basins. The breadth of the inferred fluvial system was at least 1200 km, much wider than any modern fluvial system on Earth. Zircon ages show mainly local provenance at the base of the Shaler Supergroup, with the gradual up-section introduction of more distal sources that closely match the ages of magmatic rocks characterizing the Grenville and Granite-Rhyolite Provinces of eastern and southern Laurentia, respectively. Two episodes of siliciclastic influx, represented by the Escape Rapids Formation and the younger Nelson Head Formation (Rae Group), correspond with periods of uplift and erosional unroofing that would have followed the two main phases of the Grenvillian orogeny, the Shawinigan (ca 1200–1140 Ma) and the Ottawan (ca 1090–1050 Ma). Carbonate rocks of the intervening Mikkelsen Islands Formation, and possibly correlative Tabasco Formation of the Mackenzie basin, are interpreted to record an interval of tectonic quiescence and subdued siliciclastic influx. Our work establishes a detrital zircon reference for late Mesoproterozoic–Neoproterozoic strata of northern Laurentia that is comparable with that of coeval sequences from the circum–North Atlantic region. These strata could have been deposited in similarly large interior basins, possibly connected to the Amundsen-Mackenzie and Borden basins. Such a paleogeographic scenario was likely only possible during a time of continental aggregation (e.g., supercontinent Rodinia), when orogenic belts extended between adjacent continental blocks and huge basins lay within the supercontinent’s interior.