Díaz was described by historical writer George Parker Winship as "a hard worker and skillful organizer and leader", who "inspired confidence in his companions and followers, and always maintained the best of order and of diligence among those who were under his charge".[1]: p.

When in 1539, Fray Marcos de Niza returned from Pimería Alta reporting he had seen the fabled cities of Cibola, Viceroy Antonio de Mendoza sent Díaz as the leader of a small expedition preliminarily to determine if reports by Fray Marcos were true.

In July 1540, Díaz was sent to take the now-mistrusted and hated Fray Marcos back to Mexico and (say some reports) to take over leadership of the outpost at San Geronimo (or Hieronimo) in the valley of Corazones, now Ures, Sonora, and from there to attempt contact with the fleet of Hernando de Alarcón, which was to be the maritime arm of Coronado's expedition.

Near the confluence of what is now the Colorado and Gila Rivers he learned from the natives that Alarcon had departed, but had left a cache of supplies and correspondence, which he located.

The message basically stated that Francisco de Alarcón reached this place in the year '40 with three ships, having been sent in search of Francisco Vazquez Coronado by the viceroy, D. Antonio de Mendoza; and after crossing the bar at the mouth of the river and waiting many days without obtaining any news, he was obliged to depart, because the ships were being eaten by worms.[1]: p.

While on this expedition (reports vary, some saying it ended further exploration, others saying it occurred while on the return) Díaz accidentally suffered a mortal wound.