Remnant wood found above the present-day altitudinal limit of trees provides a record of treeline shifts in response to past variations in climate. ¹⁴C dating and wood identification of remnant trees from Windy Ridge, a ridgeline in the northwestern Uinta Mountains of northeastern Utah, show treeline ∼100 m higher than today between ∼730 and 500 calibrated ¹⁴C years before present (cal yr BP; ∼1220–1450 CE). This treeline shift coincides with the Medieval Climatic Anomaly (MCA), suggesting that warm temperatures allowed limber pine (Pinus flexilis) and Engelmann spruce (Picea engelmannii) to establish above the current treeline during that interval. However, 2 of the remnant trees (limber pine and Douglas-fir [Pseudotsuga menziesii]) date to ∼175 cal yr BP (∼1775 CE), well after the end of the MCA and beginning of the Little Ice Age. This may indicate that some trees that established during the MCA were able to persist for centuries under colder conditions. Comparison of the Windy Ridge ¹⁴C dates with those of remnant trees from elsewhere in the western United States suggests that the timing of, and controls on, past treeline dynamics varied across the region.