Shortly after the initial rise of atmospheric oxygen in the Paleoproterozoic Era, a major perturbation occurred in the global carbon cycle, which is manifested as a long-lived positive carbon isotope excursion recorded in ∼2.22–2.06 Ga carbonate rocks, known as the Lomagundi Event. Beyond its significance for evolving seawater composition, this geochemical event can be used as an indirect age marker in Paleoproterozoic sedimentary successions. Documenting further occurrences of this event in other Paleoproterozoic carbonate rocks confirms that the event was global and reflects ambient seawater composition. This event, however, has only been documented in two successions in Canada, despite the ubiquity of Paleoproterozoic-aged rocks on the Canadian Shield. Our study focuses on metacarbonate rocks from the Paleoproterozoic-aged Murmac Bay Group on the southwestern margin of the Rae craton in northern Saskatchewan. Measured δ13C values (up to 7.8‰) fall within the range of the Lomagundi Event, but most values are relatively low, suggesting CO2 loss (decarbonation) altered the δ13C values. Stable carbon isotope data coupled with major element geochemical data allowed us to account for the degree to which the values have changed due to metamorphic CO2 loss. We also compare δ13C values from micro-drilled (dolomite and calcite phases) with whole-rock δ13C values from the same samples to characterize the δ13C composition of pre-metamorphic carbonate minerals. Both measured and corrected δ13C values place the Murmac Bay Group metacarbonates within the range of values that characterize the Lomagundi Event and indicate, for the first time, the presence of the Lomagundi Event on the Rae craton.