Mexico City is the capital of Mexico.
Sark was sent to Mexico City to deploy a neutron bomb in the area in order to kill Alia Gizabi, the ex-wife of Ahmad Kabir. Sydney and Vaughn were sent there as well to rescue Gizabi before the bomb went off, which ended in success. However, 62 other civilians were incinerated when the bomb went off.
Sydney was sent to Mexico City again to monitor a meet between Boris Oransky and Julian Sark. During the meet, Oransky spotted Sydney and tried to flee, but wound up taking a hostage in the process. With the help of Marshall, Sydney was able to free the hostage and defeated all of Oransky's guards, although Oransky escaped even though getting shot in the leg.