From Halopedia, the Halo wiki
Crash-landing on Halo
After boarding a lifeboat, he survived the crash landing on Installation 04, becoming the infantryman at the receiving end of most of the then-Staff Sergeant Johnson's shouting and yelling. During the events of the crash landing, he fought alongside the Master Chief, until he was evacuated by Echo 419.
Encounter with the Flood
Later on, he was part of Fire Team Charlie, which was led by Captain Jacob Keyes and Sergeant Johnson, to find a supposed Covenant weapons cache that a captured Sangheili, 'Qualomee, had uncovered and disclosed to them in passing.
Instead, the "cache" turned out to be a Flood containment facility. Fire Team Charlie was immediately ambushed and, save for Sergeant Johnson, was assimilated into the Flood and turned into Flood combat forms. Though Mendoza held off the Pod infectors for a while, unlike his comrades, he was also later assimilated into a combat form. Shortly before he was infected, he attempted to flee the area, against Keyes' and Johnson's orders.
A few hours after infection, John-117 found Mendoza in the structure and attempted to communicate with the Marine, unknown that his consciousness was already taken over by the Pod infector, which is because John-117 hadn't seen a human Flood combat form until then. The infected Mendoza attacked the Spartan and took three entire shells from the Master Chief's Shotgun to halt him, with the results both "spectacular and disgusting". As the corpse-like horror came apart, the Spartan saw that one of the small, spherical creatures had taken up residence inside the Marine's chest cavity and seemed to have extended its tentacles into other parts of what used to be Mendoza's body.
List of appearances