Mountain hemlock growth chronologies were used to reconstruct the mass balance of South Cascade Glacier, an alpine glacier in the North Cascade Range of Washington State. The net balance reconstruction spans 350 years, from 1659 to 2009. Summer and winter balances were reconstructed for 1346–2009 and 1615–2009, respectively. Relationships between mass balance and winter precipitation, temperature, the Pacific Decadal Oscillation index, and the El Niño Southern Oscillation index indicate that these influence glacier balance at various temporal scales. Above-average net, summer, and winter mass balance occurred mainly in 1690–1710, 1810–1820, 1845–1860, 1865–1890, and 1975–1990, and below-average balance periods include 1680–1690, 1790–1810, 1820–1840, and 1930–1960. Above- and below-average reconstructed mass balances at South Cascade Glacier were concurrent with similar periods from other glacier balance reconstructions in the Pacific Northwest region of North America. Agreement among these records suggests that changes in South Cascade Glacier mass balance are good indicators of regional balance fluctuations, and glaciers in the Pacific Northwest are responding similarly to regional external forcings. The current rate of decline, from 2000 to 2009, in the reconstructed balance record has been faster than any decline in a century. This decreasing trend is projected to continue with increasing temperatures, and will likely affect glacier-influenced water resources in the Pacific Northwest.