Argillochelys antiqua was a small early sea turtle with a relatively narrow, strongly ossified shell. Its carapace was roof-shaped in cross-section and carried a distinct median keel, giving the animal a more angular outline than the broad, smoothly rounded shell associated with many living sea turtles. Like other early chelonioids, it was adapted for marine swimming and probably spent much of its life in warm coastal waters.

The skull and jaws show a moderately developed palate and a sharp biting margin rather than an exceptionally broad crushing surface. These features are consistent with grasping and processing relatively small marine prey or other soft to moderately resistant food, although its exact diet is not directly known. The species is unusually well represented for an Eocene sea turtle by both skull and shell material, so its head and carapace can be considered together when reconstructing the animal.