Gerhardt is a former Army Ranger and is now the assistant team leader of Sergeant Major Jonas Blane's Alpha Team of the 1st Special Actions Group (official cover as the "303rd Logistical Studies Group"). He is competent in Italian and German; and while competent at submarine lock in/lock-out procedures, is slightly claustrophobic. He and his wife, Tiffy Gerhardt, have two daughters.
Gerhardt has worked as an operator with Blane and his team on a multitude of missions, including the recovery of a radioactive component of a Chinese satellite, the rescue of the drug minister of Mexico, the assassination of a terrorist financier in Spain, the bugging of an Iranian official's car at their embassy in Beirut, the recovery of seven Stinger Missiles from a Brazilian arms dealer, and the apprehension of a Serbian war criminal in the Balkans.
During the 3rd episode of Season 1, 200th Hour, Mack is accidentally shot during a live fire training course by Bob Brown. It is later discovered that the incident occurred because Mack had entered Brown's sector.
At the end of season 1, the apprehended war criminal, a Yugoslavian General named Razlan Dragovich, got a good look at Blane and suspected him of firing the shot that killed Dragovich's wife. When Dragovich escaped UN custody, he tracked Blane back to Fort Griffith, the Alpha Team's headquarters. When the team and their families went off post to celebrate the marriage of Unit commander Colonel Tom Ryan, Dragovich and his men attacked. They killed several restaurant employees and injured both Blane and the new Mrs. Ryan.
A few months later (during Season 2), Gerhardt prepared to leave the Unit at the behest of his wife, but Tiffy decided to rescind her request and ripped up Gerhardt's transfer papers. Since then, he has helped take down the world's foremost weapons dealer, and helped take out a rebel leader in Paraguay.
In the episode Johnny B. Goode, Mack and the team are in Iran to determine its Weapons of Mass Destruction capability. Mack kills a herder who would have compromised the mission, as well as their local contact after an explosion's debris pinned her to the ground to spare her from imminent capture, torture and death. When he returned home, he and Tiffy bickered, and Mack went to a bar to blow off steam. While there, he assaulted a civilian before being guided out by Crystal Burns, the fiancée of a former Alpha Team staffer. Crystal offered to drive Mack home, and commented on how screwed up the both of them were, as Crystal's fiancée had recently suffered an extensive traumatic burn injury and she was incapable of standing by him. Crystal then kissed Mack. He resisted at first, but gave in, and the two had a one night stand in the front seat of her new Mustang.
Later on, Crystal threatened to divulge the affair to Tiffy if Mack didn't continue the affair. This did not faze him, and he ordered her to go back to Jeremy and to stay away from him.
At the end of the 2nd Season, Gerhardt and Williams are placed in prison for unnamed actions which they committed on a mission. Brown manages to escape this fate by becoming a double agent to the CIA; while Grey disappears and Blane goes into hiding. At the early of the 3rd Season, Gerhardt is brutally tortured by the CIA, but later released soon after other team members threatened members of CIA.