Bernal Díaz del Castillo was born around 1496 in Medina del Campo, a prosperous commercial city in Castile. His parents were Francisco Díaz del Castillo and María Díez Rejón. His father was a regidor (city councilor) of Medina del Campo which provided the family with some prominence. Díaz had at least one older brother and they attended school together, learning to read and write. Bernal Diaz was intelligent and later showed a knack for languages, learning to speak the native dialect in Cuba, Nahuatl in Mexico, and the Cakchiquel language of the Guatemalan natives.
In 1514, when Díaz was about eighteen years old, he left home to join an expedition to the New World led by Pedrarias Dávila. It was the largest fleet yet sent to mainland America, consisting of 19 vessels and 1,500 persons. Díaz served as a common foot soldier and hoped to make his fortune but when they reached Darien in present-day Colombia, they were quickly overcome by famine and an epidemic that killed more than half of the settlers.  Many of the colonists grew discouraged and looked elsewhere for new opportunities; some returned to Spain while others sailed to Hispaniola or Cuba.
In 1516, Diaz sailed to Cuba with about 100 other soldiers looking for a share of the gold and native laborers that were said to be found on the island. They discovered that gold was scarce and the native labor was in short supply, leading Díaz, in 1517, to join an expedition organized by a group of about 110 disaffected soldiers and settlers to "discover new lands". They chose Francisco Hernández de Córdoba, a wealthy Cuban landowner, to lead the expedition. It was a difficult venture and, after sailing from Cuba for 21 days, they came across the Yucatán coast in early March 1517, on the Cape Catoche.
On March 4, 1517, the Spanish had their first encounter with the Yucatán natives who came to meet them on five or perhaps 10, depending on the version/translation of his work, large wooden canoes. The next day, the Spaniards disembarked, invited by the natives who wanted to show them their village. They were ambushed but managed to retreat, after killing 15 locals and having 15 wounded, 2 of whom later died. Upon leaving, the Spaniards captured 2 natives who would be translators in future expeditions. The Spanish almost died of thirst and sailed to Florida in search of potable drinking water. As they were digging a well on the beach, the Spaniards were attacked by locals. During this fracas, one Spaniard was captured by the native Floridians while the Spanish killed 22 natives. The Spanish managed to make a retreat but were also able to gather some water. They returned to Cuba, all of them severely wounded. The captain, Francisco Hernández de Córdoba, and other soldiers died shortly after making it back to Cuba.
Nevertheless, Díaz returned to the coast of Yucatán in April 1518, in an expedition led by Juan de Grijalva, with the intent of exploring the lands. Upon returning to Cuba, he enlisted in a new expedition, this one led by Hernán Cortés.
In this third effort, Díaz took part in the campaigns against the Mexica, later called the Aztec Empire. By this time, he was a highly experienced member of Hernán Cortés's expedition. During this campaign, Díaz spoke frequently with his fellow soldiers about their experiences. These accounts, and especially Díaz's own experiences, served as the basis for the recollections that Bernal Díaz later told with great drama to visitors and, eventually, a book entitled Historia verdadera de la conquista de la Nueva España (English: The True History of the Conquest of New Spain). In the latter, Díaz describes many of the 119 battles in which he claims to have participated in, culminating in the defeat of the Aztecs in 1521. This work also claims to describe the diverse native peoples living in the territory renamed New Spain by the Spaniards. Bernal Díaz also examines the political rivalries of Spaniards, and gives accounts of the natives' human sacrifices, cannibalism and idolatry, which he claims he witnessed first-hand, as well as the artistic, cultural, political and intellectual achievements of the Aztecs, including their palaces, market places and beautifully organized botanical and zoological gardens. His account of the Mexica along with that of Cortés are first-person accounts recording important aspects of Mesoamerican culture. True History remains one of the best accounts we have of Mexico at the time of the conquest, but its purpose and style betrays some of the biases that appear in this so-called truthful history. Bernal Díaz's account has not been fully utilized as a source for conquest-era Mesoamerican culture.
As a reward for his service, Díaz was awarded an encomienda by Cortés in 1522. That was confirmed and supplemented by similar awards in 1527 and 1528. In 1541, he settled in Guatemala and, during the course of a trip to Spain, was appointed regidor (governor) of Santiago de los Caballeros de Guatemala, present-day Antigua Guatemala, in 1551.