From early youth he navigated vessels for the Portuguese government to the coast of Africa and the East Indies, and he went in 1515 to Panama, where he acquired the reputation of a skilled pilot.
When in 1521 Gil González Dávila received a commission from the crown of Spain to explore and conquer the Mar del Sur (South Sea) Pacific coast west, and north, of Panama, he constructed four vessels with timber that was transported with immense labor across the isthmus from the Atlantic.
Gonzalez landed on the coast in the Bay of San Vicente (Current Caldera in Costa Rica) with part of his forces to explore the country, and sent Niño further north to discover a passage to the Atlantic of which the Indians had told him.
Niño followed the coast to latitude 17º 50' north, and, finding no passage, returned to the south, where he found the Adelantado besieged by an army of 4,000 Indians, whom he helped to disperse.
They then continued to explore the coast of Cape Blanco, the Bay of Papagayos, Possession River, and a gulf which they called Fonseca, in honor of the president of the council of the Indies.