He is known primarily for his extensive work, Crónicas del Perú (The Chronicle of Peru), which has been a source of knowledge for centuries for different disciplines such as history, philology, geography, biology, anthropology, botany and zoology.

In 1536, in Córdoba, at 16 or 18, Cieza de León was greatly surprised to learn of the discovery of the land of the Incas and so decided to go to Seville to embark on his journey to South America, to see for himself the artifacts of precious metals which had been brought to Spain from Cajamarca.

He complained about the state of affairs, writing: I do not approve of the overthrow of power in any way, but I still mourn the extortion and ill-treatment inflicted by the Spaniards on the Indians, enslaved by cruelty, despite their nobility and the high dignity of their people.

The third part of Cieza de León's Crónicas del Perú, which examined the discovery and conquest of Peru by the Spaniards, was considered by historians to be lost.

Pedro Cieza de León's historical works led the historian Raúl Porras to coin him the phrase: "the premier chronicler of the Indies."

Though his works are historical and narrate the events of the Spanish conquest of Peru and the civil wars among the Spaniards, much of their importance lies in his detailed descriptions of geography, ethnography, flora and fauna.