The dynamic response of Greenland tidewater glaciers to oceanic and atmospheric change has varied both spatially and temporally. While some of this variability is likely related to regional climate signals, glacier geometry also appears to be important. In this study, we investigated the environmental and geometric controls on the seasonal and interannual evolution of Helheim and Kangerlussuaq Glaciers, Southeast Greenland, from 2008 to 2016, by combining year-round, satellite measurements of terminus position, glacier velocity, and surface elevation. While Helheim remained relatively stable with a lightly grounded terminus over this time period, Kangerlussuaq continued to lose mass as its grounding line retreated into deeper water. By summer 2011, Kangerlussuaq’s grounding line had retreated into shallower water, and the glacier had a { extasciitilde}5-km-long floating ice tongue. We also observed seasonal variations in surface velocity and elevation at both glaciers. At Helheim, seasonal speedups and dynamic thinning occurred in the late summer when the terminus was most retreated. At Kangerlussuaq, we observed summer speedups due to surface-melt-induced basal lubrication and winter speedups due to ice-shelf retreat. We suggest that Helheim and Kangerlussuaq behaved differently on a seasonal timescale due to differences in the spatial extent of floating ice near their termini, which affected iceberg-calving behavior. Given that seasonal speedups and dynamic thinning can alter this spatial extent, these variations may be important for understanding the long-term evolution of these and other Greenland tidewater glaciers.