Archelon was a slow mover and found most of its food drifting near the sea surface. It had little need to dive deep except when hibernating on the seabed. It was an omnivorous grazer, sweeping up drifting fish, jellyfish and dead carrion as well as plants. Its sharp, powerful beak could break open shelled animals such as ammonites.
Archelon's huge flippers suggest it was a long distance swimmer happiest in the open ocean. It would never be alone, as its huge size attracted a squadron of hangers-on such as juvenile fish as well as barnacles and parasites. Archelon couldn't withdraw its head or flippers inside its bony shell for protection so, despite its size, it was an easy target for large predators.
Like modern turtles, it laid eggs by burying them in sandy beaches under cover of darkness. Its nearest living relative is the world's largest turtle, the leatherback.