The occurrence of mantle plumes in the geologic past is hypothesized to be marked by voluminous basaltic volcanism and topographic and gravitational anomalies. Missing from these identifying characteristics is a direct measurement of the elevated mantle temperatures associated with an upwelling channel from the deep mantle. To assess the extent of plume heating in the 1.1 Ga Midcontinent Rift System (North America), we present U-Pb thermochronologic evidence for a ca. 1.1 Ga sublithosphere heat source near Attawapiskat, Canada, >600 km from the inferred plume center. Apatite and rutile U-Pb cooling dates from middle to lower crustal xenoliths exhumed in the Jurassic Victor kimberlite record a thermal history >2.5 b.y. in duration. Shallow amphibolite and gabbro yield Archean to Paleoproterozoic dates with high U-Pb discordance, consistent with middle crust cooling prior to 1.1 Ga. Deeper garnet-bearing samples yield younger dates with low U-Pb discordance. Replicating these data with models reveals a thermal history in which the extent of heating corresponds with sample depth, an observation consistent with heating from below. Thermochronologic data are best fit by model simulations in which the Attawapiskat lithosphere experienced a ca. 1.1 Ga heating event triggered by partial lithosphere removal and mantle temperatures >200 °C in excess of that of ambient mantle, consistent with a model of ∼100 m.y. plume head residence beneath the Attawapiskat region.