Passive tectonic margins, the transition between continental and oceanic lithospheres, represent a quiescent stage in plate tectonics where active lithospheric deformation along the boundary has ceased. Despite these being globally ubiquitous features, little is known about the mantle dynamics associated with passive margins owing to the difficulty of obtaining margin-crossing seismic data sets. Combining ocean-bottom seismic data from the recent Eastern North American Margin Community Seismic Experiment and onshore data from the EarthScope Transportable Array and other deployments, we assimilate a seismic data set spanning the passive eastern North American margin (ENAM). We infer mantle dynamics along the margin using observations of shear wave splitting. Offshore, we observe margin-parallel fast splitting directions that are inconsistent with either plate-driven flow or paleo-spreading frozen-in anisotropy. Instead, splitting beneath the oceanic plate likely indicates margin-parallel mantle flow. Onshore measurements east of the craton edge show predominantly null splitting, suggesting vertically oriented flow. The heterogeneity of shear wave splitting across the ENAM suggests a unique dynamic response of the upper mantle to the presence of the passive margin. We propose that a margin-wide geodynamic process (e.g., pressure- and/or density-driven flow or edge-driven convection) drives dynamics beneath both the onshore and offshore regions.