He is depicted as an African-American mechanic and engineer who is recruited into the newly rebuilt S.H.I.E.L.D. by Phil Coulson after the events of Captain America: The Winter Soldier. He is also old friends with Barbara "Bobbi" Morse. In the episode "One of Us," it is revealed that he and Bobbi are secretly associated with "the real S.H.I.E.L.D." In the episode "One Door Closes," it was revealed that Mack was on the S.H.I.E.L.D. aircraft carrier Iliad at the time when the Hydra infiltrators made their move. He and Robert Gonzales were saved by Bobbi Morse and Isabelle Hartley. Mackenzie, though close friends with the Inhuman S.H.I.E.L.D. agent Daisy Johnson, is quite distrustful of Inhumans. In the episode "Closure," Phil Coulson appoints Mack as interim Director of S.H.I.E.L.D. The episode "Watchdogs" introduced Mack's brother Ruben (portrayed by Gaius Charles) who had a different opinion of the Inhumans than his brother. Ruben later discovered his brother's job as S.H.I.E.L.D. even at the time when the Watchdogs raid Mack's house upon mistaking him for an Inhuman. With help from Mack, Ruben was able to defeat some of the Watchdogs where one of them wounded Mack. Upon meeting Daisy Johnson at the time when Mack was taken to S.H.I.E.L.D.'s infirmary while the wounded Watchdogs members are transferred to the Advanced Threat Containment Unit, Ruben mentioned to her that he was originally nicknamed "Mack" before his brother was called that by Daisy and the rest of Phil Coulson's group.