New shear wave splitting measurements (n = 76) from the Mid-Atlantic section of North America, when combined with previously reported measurements, provide improved clarity to patterns of seismic anisotropy across the region, including a rotation of fast polarization directions (ϕ) from east-west to northeast-southwest in Pennsylvania (USA). We attribute the patterns in ϕ to frozen-in (i.e., fossil) anisotropy in the lithospheric mantle. Most ϕ directions parallel the strike of the Appalachian orogen, except in parts of New England, where ϕ directions are oriented perpendicular to both the strike of the orogen and continental margin. The ϕ directions parallel to the strike of the Appalachian orogen, including those that rotate from east-west to northeast-southwest in Pennsylvania, are consistent with the expected orientation for fossil anisotropy developed during the Appalachian orogenic cycle. The ϕ directions in New England, perpendicular to the continental margin, are consistent with the expected orientation for fossil anisotropy developed during the breakup of Pangea, and ϕ directions at locations west of the Appalachian Mountains in areas not affected by the Appalachian orogeny can be explained by fossil anisotropy developed during the Proterozoic suturing of the Granite-Rhyolite Province and Elzevir block to the eastern margin of Laurentia.