Metriorhynchus was a versatile hunter, eating everything from the slow moving Ammonites and Belemnites to faster prey such as the giant fish Leedsichthys and even pterosaurs. It probably did this by resting with its head just under the surface so that only its nostrils remained above the surface. Then when a pterosaur strayed too close, it would give a powerful stroke with its tail and lunge out of the water, clasping the unfortunate flying reptile in its jaws.
Despite its powerful hunting ability, Metriorhynchus was relatively defenceless against other larger hunting reptiles like the liopleurodon. Unlike modern crocodiles, Metriorhynchus lost most of its armour in order to be able to swim faster. It was so adapted to life at sea that it probably only returned to land to mate and to lay its eggs. It was not very graceful when out of the water and would have returned to the sea immediately after laying its eggs. The young would have hatched on their own, making a hazardous journey down the beach to the sea.