[1][2] His parents were José Joaquín Tristán del Pozo y Carassa and María Mercedes Moscoso Pérez Oblitas, members of the Arequipa Creole high society.

He studied in the city of Cuzco, he was later sent by his parents to Spain where he served for a time as a Midshipman in the Royal Navy, in 1791 he returned to Peru after being appointed colonel of the Majes Valley militias.

He repressed the Yungay uprising, defeating in Irupana the caudillo Manuel Victorio García Lanza who, together with Gabriel Antonio Castro, he pursued until the Indians under his command managed to capture them and cut off their heads, which were exhibited by Tristán in November 1809 in the town of Chulumani.

However, when the rebel Army of the North advanced towards Potosí, on 16 November 1810, then-Governor Tristán signed the act of adhesion to the government of Buenos Aires and three days later issued a patriotic proclamation.

Despite his lack of military qualities, San Martín entrusted him with the command of an important division of his army that in 1822 was completely destroyed in the Battle of Ica by the Spanish general José de Canterac.