Lesothosaurus diagnosticus was one of the most primitive and earliest ornithischians, or "bird-hipped" dinosaurs. Ornithischians were one of the two major groups of dinosaurs and included such familiar species as the armor-plated Stegosaurus and Ankylosaurus and the three-horned Triceratops.

Lesothosaurus was small, agile, and bipedal, bearing seemingly little resemblance to the gigantic, quadrupedal ornithischians that came later. But physical features, such as hip structure and jaws and teeth designed to chew plants, link Lesothosaurus to the later ornithischians.

Lesothosaurus evolved in what is now southern Africa in the early Jurassic period, just over 200 million years ago. It gets its name from the African country of Lesotho, where early fossils were uncovered.

The lifestyle of Lesothosaurus has been compared with that of modern-day gazelles, which browse on low-growing vegetation and nervously scan their surroundings for danger, fleeing when a predator approaches too near.

Lesothosaurus had a small head with very large eyes, short front legs with grasping hands, and an elongated body and tail about three feet (one meter) long. Its jaws were lined with small, serrated teeth for shredding plants and, like most ornithischians, its mouth was tipped with a pointed beak. Long, muscular legs, hollow bones, and clawed feet would have allowed this lightweight dinosaur to outsprint or outmaneuver most of its predators.