In the last two decades, Pine Island Glacier (PIG) experienced marked speedup, thinning, and grounding-line retreat, likely due to ice-shelf basal melt and marine ice-sheet instability. To better understand these processes, we analyzed 2008–2010 and 2012–2014 in-situ GPS records for PIG to constrain surface mass balance, firn compaction, and basal melt. We computed time series of horizontal velocity, strain rate, antenna height, surface elevation, and Lagrangian elevation change (Dh/Dt). The antenna height time series show a surface elevation increase of ~ 0.7–1.0 m/yr, which is consistent with model estimates for surface mass balance (SMB) of ~ 0.7–0.9 mwe/yr and ~ 0.7–0.8 m/yr downward velocity due to firn compaction. An abrupt ~ 0.2–0.3 m surface elevation decrease, likely due to surface melt, is observed during a period of warm atmospheric temperatures from December 2012 to January 2013. Observed Dh/Dt for all PIG shelf sites is highly linear, with trends of −1 to −4 m/yr and residuals of < 0.4 m. Corresponding basal melt rate estimates range from ~ 10 to 40 m/yr, in good agreement with those derived from ice-bottom acoustic ranging, phase-sensitive ice-penetrating radar, and high-resolution stereo DEM records. The GPS and DEM records document higher melt rates within and near features associated with longitudinal extension (transverse surface depressions, rifts). Basal melt rates for the 2012–2014 period show limited temporal variability, despite significant changes in ocean heat content, suggesting that sub-shelf melt rates may be less sensitive to ocean heat content than previously reported, at least for these locations and time periods.