We employ high-resolution, high-precision, helicopter-borne gravity observations of the Northern (NPI) and Southern Patagonia Icefields (SPI), South America, to infer ice thickness and bed topography using a three-dimensional model constrained by Fjord and lake bathymetry, and a land-ice mask. The results reveal thicker ice than the reflecting horizon of radar sounders, with 1 km deep ice for Glaciar San Rafael and Colonia (NPI) and 1.5 km deep ice for Glaciar Occidental (SPI). These bedrock troughs channelize fast motion of ice from the plateaus. Combining ice motion and thickness, we calculate balance accumulation levels of 3–6 m/yr water equivalent for the plateaus. Bed elevation remains below sea/lake level 15–20 km inland for Jorge Montt and O’Higgins, which favors retreat, and is at sea level for San Rafael, which halted its retreat. The results demonstrate the utility of airborne gravity surveys for providing critical data on ice volume, bed elevation, and balance accumulation of temperate ice masses.