He is best known for discovering the fossil of a female hominin australopithecine known as "Lucy" in the Afar Triangle region of Hadar, Ethiopia.

[2] Lucy was discovered in Hadar, Ethiopia on November 24, 1974, when Johanson, coaxed away from his paperwork by graduate student Tom Gray for a spur-of-the-moment survey, caught the glint of a white fossilized bone out of the corner of his eye and recognized it as hominin.

The whole team including Johanson concluded from Lucy's rib that she ate a plant-based diet and from her curved finger bones that she was probably still at home in trees.

The subsequent discovery of several more skulls of similar morphology persuaded most palaeontologists to classify her as a species called afarensis.

[4] AL 333, commonly referred to as the "First Family", is a collection of prehistoric homininid teeth and bones of at least thirteen individuals that were also discovered in Hadar by Johanson's team in 1975.