Justo was born on August 7, 1829, at Moquegua as the son of Don Vicente Fernández Dávila Pomareda and Doña Teresa Herrera Maura.

On 1879, he was made the Prefect of Tarapacá before the Peruvian involvement in the War of the Pacific began as Mariano Ignacio Prado appointed him as the commander of the Vanguardia Division which was previously commanded by General Manuel G. De la Cotera.

[1][2] This was due to several personnel changes in the command of the divisions during the month of May and after Belisario Suárez was appointed chief of the general staff, replacing Colonel Bustamante, of which Dávila himself supported.

La Cotera grew angry because of this and refused the position of chief of general staff and went to Lima alone with his assistant.

He was replaced by Colonel Don Justo Pastor Dávila who was described as a courageous, impetuous and an extremely active chief.