Garcilaso was the third son of Alonso de Hinestrosa de Vargas and Blanca de Sotomayor. He served with Pedro de Alvarado, and participated in the conquests of Hernán Cortés, first in Mexico and later in Guatemala. In 1534, he left for Peru. After arriving in Venezuela, he marched to Quito and later joined the army of Francisco Pizarro. After receiving orders to conquer the Cauca River valley, he abandoned the attempt to colonize the San Mateo Bay, returning to Lima with his eighty men to encounter Manco Inca Yupanqui.