With large clavicles and interclavicle and small scapulae, Leptocleidus resembled the Early Jurassic Rhomaleosaurus and members of the Cretaceous family, Polycotylidae.

The Leptocleidus' triangle-shaped skull had a crest running from a ridge on the end of the nose to the nasal region.

Differing from pliosaurids, Leptocleidus had single-headed cervical ribs and a deep depression in the centra of the neck vertebrae.

[3] Leptocleidus, unlike many plesiosaurs, lived in shallow lagoons and likely visited brackish and fresh water systems (such as the mouths of large rivers).

This led Arthur Richard Ivor Cruickshank to infer that this movement to fresh water was an attempt to flee larger plesiosaurs and pliosaurs.