Eosphargis gigas was a large early leatherback turtle and an important stage in the evolution of the highly specialised dermochelyid body plan. Its shell was already reduced compared with the solid carapace of ordinary hard-shelled sea turtles, but the reduction was not yet as extreme as in later leatherbacks. Broad, ridged midline shell bones and the form of the pectoral skeleton show an animal built for sustained marine swimming.

Large humeri and shoulder-girdle elements indicate powerful forelimb propulsion, so Eosphargis was probably an active coastal or open-water swimmer rather than a bottom-walking turtle. Its diet is not directly preserved. Although the living leatherback is strongly associated with gelatinous prey, there is no good reason to assume that this specialised feeding ecology had already evolved unchanged in Eosphargis. London Clay material includes skull, shell and substantial limb and shoulder bones.