After the other employees hid inside the employee tunnels during the initial attack of the Great War, Mitchel was nowhere to be found. Eventually he turned up with a semi half truck full of guns, explosives, food, medicine, and chemicals. He said he found all the items around Nuka-World, but his shirt was covered in blood.
During the very first attack on the group, Evan Barnes was almost killed, but was saved by Mitchel. Mitchel eventually turned into a feral ghoul, like most of the group.