Zack was a SOLDIER 1st Class from Gongaga. While stationed in Midgar, he meets and dates Aerith. Five years prior to the start of the game, he is sent, along with Cloud, Sephiroth and some other soldiers, to investigate reports of monsters disrupting life for the workers at the Mt. Nibel Mako Reactor. After Sephiroth goes insane, Cloud and Zack are taken prisoner in the basement of the Shin-Ra Mansion and used as test subjects for Hojo's efforts to produce a Sephiroth "clone".

Zack breaks out and carries the incapacitated Cloud with him during their escape. Zack's ability to escape from his imprisonment is due to the fact that he -- being an individual full of self-confidence -- was unencumbered by the potentially negative side-effects of Mako treatment (known as "Mako Poisoning", in which the memories of the deceased within the Spirit Energy/Mako cause those with low self-confidence to become lost and confused within their own minds) and had handled the treatment before when being infused with Mako as part of the procedure to become a member of SOLDIER. He gives Cloud some of his clothes and gets them transport to Midgar, where he hopes to become a mercenary. However, upon arriving to the outerskirts of Midgar, they are discovered by Shin-Ra soldiers. Zack is shot to death, while Cloud, too sick to move, is simply left to die on his own. Subsequently, Cloud takes Zack's persona and Buster sword and, for much of the game, acts as though Zack's life was actually his own, having what appears to be severe headaches and mental pain when faced with a situation that would conflict with his life as Zack.