Pedro was the first-born of 15 brothers of very poor background, and he started his studies at the first school opened in rebuilt Chillán.

He did not become involved in the rebellion as the event surprised him in Valparaíso; much to the contrary, he embraced the pro-government side, which won him a promotion to the rank of Lieutenant.

During the Revolution of 1859, he supported the government of Manuel Bulnes and participated soon in the siege of Talca and, in the North, in the Battle of Los Loros.

At this time, he married his cousin, Juana Marchant, and like most of the officials in the Araucanía he was awarded land, in his case in the Mulchén area.

The feat was heroic, but it was marred by the lack of control over the assaulting troops, that led to a widespread killing of the surviving Peruvian soldiers and the citizens of the already surrendered city, which was looted.

A military man until the end, Pedro Lagos also had a valuable participation in the Lima Campaign, where he fought at the battles of San Juan and Miraflores.

In March 1881, he was named Commander in Chief of the Occupation Army to replace General Baquedano, who returned to Chile.

His short stint as head of the Chilean forces in Peru was highly controversial, due to his strict disciplinary measures against the citizens of Lima.

On 18 June 1881, the Chilean Senate promoted him to Brigadier General, sent him back to Chile and appointed him chief of the Service Examination Commission.