Unrest at Mammoth Mountain over the past several decades, manifest by seismicity, ground deformation, diffuse CO₂ emissions, and elevated ³He/⁴He ratios in fumarolic gases has been driven by the release of CO₂-rich fluids from basaltic intrusions in the middle to lower crust. Recent unrest included the occurrence of three lower-crustal (32–19 km depth) seismic swarms beneath Mammoth Mountain in 2006, 2008 and 2009 that were consistently followed by peaks in the occurrence rate of shallow (≤10 km depth) earthquakes. We measured ¹⁴C in the growth rings (1998–2012) of a tree growing in the largest (∼0.3 km²) area of diffuse CO₂ emissions on Mammoth Mountain (the Horseshoe Lake tree kill; HLTK) and applied atmospheric CO₂ concentration source area modeling to confirm that the tree was a reliable integrator of magmatic CO₂ emissions over most of this area. The tree-ring ¹⁴C record implied that magmatic CO₂ emissions from the HLTK were relatively stable from 1998 to 2009, nearly doubled from 2009 to 2011, and then declined by the 2012 growing season. The initial increase in CO₂ emissions was detected during the growing season that immediately followed the largest (February 2010) peak in the occurrence rate of shallow earthquakes. Migration of CO₂-rich magmatic fluids may have driven observed patterns of elevated deep, then shallow seismicity, while the relationship between pore fluid pressures within a shallow (upper 3 km of crust) fluid reservoir and permeability structure of the reservoir cap rock may have controlled the temporal pattern of surface CO₂ emissions.