Sylar was previously a security officer in Vault 13, he worked with a number of his fellow Vault Dwellers in keeping the peace in the Vault's air tight halls. This was his job for around a year before he was sent out into the wastes.
He risked life and limb to retrieve the Water Chip, even going as far as to brave the City of the Dead to reach it. He eventually returned with the Water Chip, which was installed flawlessly, however, the overseer grew concerned about the increase in Mutant Activity, out in the wasteland.
Sylar was sent out once more to destroy the Master, the hideous mutant behind the Supermutant uprising and destroy the vats where they came from. After doing this, he returned to the Vault, only to be told that he was no longer welcome there, because of how much the outside had changed him. As a result, he was exhiled from the Vault, he slunk off, into the Wasteland and after removing his Vault Suit he vowed to never wear it again.