The Crónica de D. João I, in two parts, is the final work of Fernão Lopes. It therefore is the chronicle that shows us Lopes working in full maturity, at the height of his skills as a writer of history. Lopes composed the chronicle sometime towards the middle of the 15th century. In 1454, D. Afonso V appointed Gomes Eanes de Zurara as successor to Lopes in the post of cronista-mor. Zurara's first task was to complete the Crónica de D. João I with a third and final part, commonly referred to as the Crónica da tomada de Ceuta.

The Crónica de D. João I is remarkable, not only because it can be considered Lopes's magnum opus but also because of the political history behind it. The text recounts the rise to power and the reign of João I (r. 1385–1433), founder of the Avis dynasty, and the first Portuguese monarch elected to the throne as opposed to genealogical succession. Fernão Lopes thus faced the job of detailing how this politically anomalous chapter in Portuguese history came to be, at the same time legitimating the newly-formed dynasty with the authoritative force of prose historiography. The result is a chronicle that is self-consciously cognisant of historiographic method and claims to truth and impartiality, while also an engaging narrative in its own right, a culminating moment in the Iberian tradition of chronistic writing in the vernacular that stretches back to the 13th century. Part I of the Crónica details João's rise to power between 1383 and 1385, while Part II recounts João's reign as king to 1411.

A brief summary of the political history that is the subject of the first part will provide a context to the narrative project Lopes undertook. The series of events that led to João's occupation of the throne began in 1383 with the death of King Fernando, after which, in A. R. Disney's words, there ensued ‘the greatest dynastic crisis in Portugal since the kingdom had come into being.’ By legitimate succession, the son of Beatriz (Fernando's young heiress) and King Juan of Castile would inherit the crown; but if the marriage produced no children, then it would pass directly to Juan and Portugal and Castile would be united.