The overseer was placed in charge of Vault 111 before the Great War, though his responsibilities involved managing the vault's research and security personnel, as the residents were all placed in cryogenic stasis as per the vault's experiment. After greeting the vault's residents and ensuring they entered their cryogenic pods without becoming suspicious, the overseer settled into a routine of distributing rations, keeping tabs on the researchers, and tinkering with his pet project, the Cryolator, in his spare time.
Although the vault's staff were under orders to remain inside the vault for 180 days unless they received an all-clear signal from Vault-Tec and were evacuated, the months of waiting took their toll on the morale of the security personnel, who began to question the vault's experiment and request to leave the vault. The overseer remained aloof and deflected their questions, but as a precaution he disabled the primary door to the vault's entrance, and later posted one of the researchers outside his office to restrict the security members' access. As the 180 day mark approached and supplies dwindled, the security personnel lost their patience with the overseer's stalling and mutinied. The overseer and researchers sealed the inner door to his office and hoarded the last of the rations; the overseer believed unsealing the vault so soon after the bombs had dropped would be suicide, as the surrounding region was likely saturated with lethal levels of radiation.
In the end, the security personnel managed to circumvent the lock on the office door. They killed the overseer and the researchers, then departed the vault.