He became Garibaldi's fugleman or second in command in 2259, after it was discovered that the previous holder of that position, Jack, had shot Garibaldi, leaving him in a coma and almost killing him.
When Nightwatch first came to the station in 2259, Zack ecstatically joined primarily based on the 50 credits a week he would be paid in addition to his regular paycheck. Over the next year he slowly began to realize Nightwatch's fascistic nature, developing serious moral qualms with their actions and tactics. After witnessing firsthand what they did to a shopkeeper who had badmouthed President Clark, he agreed to help remove the organization from the station.. For his actions he was later offered the EA's Medal of Outstanding Valor, but refused to accept it.
Several years later, he either resigned or retired from Earthforce and went back to Earth for a while, but quickly got bored. Reactivating his commission in July 2280, he returned to Babylon 5, which was now in the process of being decommissioned. Resuming his post as head of security, he stayed there for six months before the station's decommissioning and destruction in January of 2281.
Zack was a good friend of Michael Garibaldi. When Garibaldi disappeared in December 30, 2260, he searched for him. On January 17, 2261 he successfully rescued him and brought him back to Babylon 5.
Zack had been fired from several jobs and had many personal problems that proved to be a problem in keeping a job for very long. When he sobered up he got a job working for his old friend. As Garibaldi said to Zack during a discussion: "You had a lot of problems before you came here, and I know, I know you had some problems after you got here, too. But did I say anything to you about it? No. You wanna know why? Because I knew, I knew you would work it out, and you always did." 
He also was a friend of Lyta Alexander. She discovered that his feelings were more than simple friendship when he tried to deter her from staying with the rogue telepaths around Byron,  but she never returned his affection.
|Main Characters (Babylon 5)|
|Commander Jeffrey Sinclair Captain/President John Sheridan Lt. Commander/Commander Susan Ivanova Security Chief Michael Garibaldi Ambassador Delenn Dr. Stephen Franklin Talia Winters Lennier Marcus Cole Captain Elizabeth Lochley Vir Cotto Lieutenant Warren Keffer Na'Toth Security Chief Zack Allan Lyta Alexander Ambassador/Citizen G'Kar Ambassador Londo Mollari|