At up to 8 metres long, Cryptoclidus was a medium-sized plesiosaur. It had a neck that was up to 2 metres long that did not seem to have been very flexible. It probably kept its bulky body away from its small head so as not to alarm potential prey. It had long pointed teeth, ideal for catching fish and squid. It had four broad paddle-shaped limbs, with which it either "flew" through the water in wave-like undulating movements, or swam like a porpoise by moving upwards on two flippers and gliding back down again on the other two.

Plesiosaurs were one of a number of different groups of marine reptiles. These included marine crocodiles and ichthyosaurs. Plesiosaurs were descendants of the land-living reptiles that had returned to the sea. Palaeontologists are divided on how closely plesiosaurs are related to these other marine groups. However, it is clear that they were only very distantly related to the dinosaurs.

Well preserved remains of Cryptoclidus, and other closely related long-necked plesiosaurs have been found throughout the world. In particular many complete Cryptoclidus skeletons have been recovered from clay quarries around Oxford and Peterborough in England.