Master Sergeant Cortez serves in the UNSC Marine Corps as an Orbital Drop Shock Trooper with the 105th Shock Troops Division, stationed aboard the destroyer UNSC Kronstadt. In 2544, he commanded a fireteam on a critical assassination mission targeting a Covenant Prophet on Heian to disrupt enemy supply lines. This operation was uniquely led by SPARTAN-II Cal-141, bypassing standard ODST command protocols.

Cortez's squad included Privates O'Brien and Checkman, and Corporal Taylor "Dutch" Miles. During atmospheric insertion via M8900 SOEIV pods through a meteor storm, Checkman's pod collided with an asteroid, killing him. O'Brien's pod landed in a sinking swamp. As Cortez and Dutch prepared a rescue, Cal-141 intervened, using her enhanced strength to free the trapped pod.

The team commenced a day-long march to the target. En route, they engaged Covenant forces, encountering Unggoy patrols and a Jiralhanae Chieftain. At ancient ruins, Cortez neutralized an Unggoy via broken neck while Cal and Dutch eliminated others. O'Brien accidentally alerted the Chieftain, provoking a confrontation where Cal saved O'Brien by hurling the Chieftain over a waterfall. The Chieftain survived and later ambushed the team at the assassination site, attacking O'Brien. Cal pushed O'Brien clear, absorbing the gravity hammer blow herself. Cortez, Dutch, and O'Brien tackled and killed the Chieftain. Cortez then removed Cal's helmet, revealing her identity to the squad. Mortally wounded, Cal ordered O'Brien to eliminate the Prophet, which he accomplished. With her final words, Cal entrusted Cortez with a data chip concerning the alien ruins for Dr. Catherine Halsey.

Post-mission, Cortez returned to the UNSC Kronstadt with Dutch and O'Brien. In the mess hall, Marine Berger mocked Cal's death, suggesting Spartans were inferior to ODSTs. Cortez and his squadmates rebuked Berger, demonstrating their hard-earned respect for Cal-141 and her sacrifice, marking a shift from their initial tensions.