A native of Ontario and the son of a stern British sergeant-major, Seth Bullock left home for good at the age of sixteen and ended up in Montana during the silver strikes of the 1860s. A natural leader, he held a number of elected positions in his early twenties and was eventually appointed Sheriff of Montana Territory.

Having heard of the big strikes in the Black Hills, Bullock has made his way to Deadwood not to prospect, but to open up a hardware franchise with his business partner and friend Sol Star. But while his intent was to become a merchant, Bullock's staunch ideas of right and wrong soon pull him back toward law enforcement. He has barely arrived in Deadwood when he teams with Wild Bill Hickok to investigate the massacre of the Metz family; not long after, he rides out after the murderer of Hickok himself.

Bullock's sense of honor also leads him to keep an eye on the interests of Alma Garret, who had sought Wild Bill's help in defending her suddenly promising gold claim. Fulfilling his promise to help her, Bullock becomes romantically involved with Alma, despite the impending arrival of his wife and children, who, Alma learns, had been the family of Bullock's now-dead brother.

Too appalled to stand by and watch the brand of justice enforced by Con Stapleton, the default Sheriff of Deadwood, Bullock has given in to his own nature (and to Swearengen's urging) and has accepted the job.