The Nipissing phase of ancestral Lakes Michigan, Huron, and Superior was the last pre-modern highstand of the upper Great Lakes. Reconstructions of past lake-level change and glacial isostatic adjustment (GIA), as well as activation and abandonment of outlets, is dependent on an understanding of the elevation of the lake at each outlet. More than 100 years of study has established the gross elevation of the Nipissing phase at each outlet, but the mixing of geomorphic and sedimentologic data has produced interpreted outlet elevations varying by at least several meters. Vibracore facies, optically stimulated luminescence and radiocarbon age control, and ground-penetrating radar transects from new and published studies were collected to determine peak Nipissing water-level elevations for the Port Huron (Lake Huron), Chicago (Lake Michigan), and Sault (Lake Superior) outlets. Contemporary elevations are 183.3, 182.1, and 195.7 m (International Great Lakes Datum of 1985 [IGLD85]), respectively. These data and published relative hydrographs were combined to produce one residual hydrograph for the Port Huron outlet that best defines the rise, peak, and rapid fall of the Nipissing phase from 6000–3500 calendar years ago. Establishing accurate elevations at the only present-day unregulated outlet of the Great Lakes and the only ancient outlet that has played a critical role in draining the upper Great Lakes since the middle Holocene is a critical step to better understand GIA and water-level change geologically and historically. The geologic context may provide the insight required for water managers to make informed decisions to best manage the largest freshwater system in the world.