It is unknown whether or not Mr. Mackey and Mr. McVey shared the same history before the phenomenon, though possible as they share the same role. Mr. Mackey is trapped inside the Grocery Store along with its patrons and other staff when the Mist appears and engulfs the town outside. On the first night of the phenomenon, Mr. Mackey uses the gas grill to barbecue chicken for the other survivors.
Mr. Mackey becomes one of Mrs. Carmody's followers after the pharmacy expedition and becomes very homicidal to David's group after he becomes a follower, acting as sort of the executioner of the congregation. He very brutally stabs Jessup when the latter is sacrificed, and is more than willing to stab an innocent child.The next morning, Mr. Mackey, along with Jim and several others, tries to stab Billy and Amanda under Mrs. Carmody's orders, but when Mrs. Carmody is shot dead, Ollie aims the gun at Mr. Mackey, who quickly throws aside his knife and backs away in terror.
Mr. Mackey is shortly after seen watching David's group drive off with a very guilty and traumatized look on his face; suggesting that not only did he snap back to reality and is guilty over what he had done, but is very worried about what will happen to him in the future if everything returns to normal.
Mr. Mackey was most likely rescued from the market as people from the market are very briefly glimpsed aboard the rescue trucks at the end.