In North America, long-term records of biodiversity and climate change that span the Last Interglacial Period (MIS 5) are exceptionally rare. Where found, they provide significant insight into how the coupling of the ocean-atmosphere system manifests in biotic and environmental records, and, as a result, how the terrestrial biosphere responds to global climate change. Despite an emerging archetype of ecosystem change in North America through MIS 6/5/4, records from high-elevation that combine multiproxy biological data with an accurate chronology are completely absent. In 2010-2011, construction on a reservoir near Snowmass Village, Colorado (USA) in the southern Rocky Mountains revealed a nearly continuous, fossil-rich, lacustrine/wetland sedimentary sequence that preserved a series of stacked ecosystems spanning ~140 to 55 ka. The combination of pollen, aquatic and terrestrial plants, invertebrates, and micro- and macro-vertebrates with a robust and directly-dated chronological framework show that high-elevation ecosystems in the southern Rocky Mountains are climatically sensitive and responded rapidly and significantly to changing environments during MIS 6/5/4. These data demonstrate both the importance and strength of known hemisphere-scale teleconnections and that climate states during the Last Interglacial Period in the southern Rocky Mountains had thresholds that led to variable local ecosystem responses to global climate trends.