Carbon isotopes are analyzed for the first time from the Ordovician–Silurian boundary interval in the Williston and Hudson Bay basins of Manitoba, revealing a prominent positive excursion. The nature of this excursion and its co-occurrence with the appearance of the Ozarkodina hassi Conodont Zone and a distinctive coral-dominated macrofauna indicate that this is the upper portion (Metabolograptus persculptus Graptolite Zone) of the globally recognized Hirnantian isotopic carbon excursion (HICE). As a result, the Ordovician–Silurian boundary is placed at a higher position than previously thought, at the disconformable Stonewall – Fisher Branch formational boundary in the Williston Basin, and probably the Port Nelson – Severn River formational boundary in the Hudson Bay Basin. Disconformities within sections suggest periods of nondeposition due to subaerial exposure as sea level fluctuated during the Late Ordovician glaciation. Latest Ordovician (Hirnantian) deposition was far more widespread on the Laurentian craton than realized previously. Age determination and correlation having unprecedented precision are attainable within and between the Williston and Hudson Bay basins, and beyond.