The guanay cormorant is similar in coloration to the rock shag, Leucocarbo magellanicus, but larger, measuring 78 cm from the tip of the bill to the end of the tail.

[2] Habitat loss and degradation and over-fishing have resulted in a steady decline of the population of about 30% from an estimated figure of three million birds in 1984.

Some taxonomic authorities, including the International Ornithologists' Union, place this species in the genus Leucocarbo.

The bird's droppings were such an important source of fertilizer to the peoples of the Andes that it was protected by Inca rulers, who supposedly made disturbing the cormorants in any way punishable by death.

The common name is an adaptation of the South American Spanish guanae, a plural of the English equivalent guano.