On 10 September 1973 Mendoza was the eighth in rank in the Chilean Police Corps, a professional military body which could have stood up to the army in defense of the Allende government.

In this position, he eventually formed part of the Government Junta that came into power in the September 11 1973 coup d'état (which coincidentally happened on Mendoza's birthday).

Of the four members of the Junta (who represented the Chilean Army, Air Force, Navy and Carabineros or National Police), Mendoza was widely considered the one who held the least amount of power and influence, even being derisively referred to as Mendocita.

Mendoza served in the Government Junta until August 2, 1985, where the public outcry from Caso Degollados led to his forced resignation.

A noted horseman, Mendoza won a silver medal in the XV Olympic Games of 1952 at Helsinki as a member of the show jumping team in the equestrian event.