Its breeding range is from around Valdivia, Chile, south to Cape Horn and Tierra del Fuego, and north to Punta Tombo in Argentina.

The rock shag was formally described in 1789 by the German naturalist Johann Friedrich Gmelin in his revised and expanded edition of Carl Linnaeus's Systema Naturae.

Latham had in turn based his description on a specimen in the Leverian Museum and the account made by Georg Forster of his visit to Tierra del Fuego in December 1774 during James Cook's second voyage to the Pacific.

It feeds close to shore, often diving at the edge of kelp beds and apparently finding small fish (predominantly cod icefishes, Patagonotothen species) sheltering among the weed.

Studies with depth gauges suggest that it is a fairly shallow diver, typically going about 5 m below the surface with few individuals diving deeper than 10 m, although its prey mainly comes from the sea floor.