Whitney is among those who answer the call of Randall Flagg, and ends up in Las Vegas. He acts as head caterer for members of Flagg's inner circle; Lloyd Henreid describes him a "fat, loud sack of shit", before conceding that the man has a real gift when it comes to food.
An ex-Army cook and former butcher, Whitney is usually dressed in cooks' whites that are dirtied from his constant activities in the kitchen. He had a flat-top haircut and gapped teeth.
He is lovers with former nightclub dancer Jenny Engstrom.
(None of these characterizations are retained in the TV miniseries, where Horgan is portrayed as a generic underling of Flagg's.)
After Flagg's control over Las Vegas begins to visibly slip, Whitney approaches Lloyd with the news that he, along with a small party of other defectors, plan on fleeing to South America before things go really bad.
When Larry Underwood and Ralph Brentner are scheduled for a public execution by dismemberment, Whitney is the only one in Las Vegas to stand up and denounce the proceedings, condemning them as barbaric. He tells the crowd "we was Americans once" that "this ain't how Americans act", taking them to task for going along with the execution because "some murdering freak in cowboy boots" willed it.
But his words fail to spark a revolt. Flagg retaliates by flicking an energy-ball at Whitney that seals his mouth shut and then his eyes. Whitney falls forward, but it is not specified whether he has been killed, or is just maimed and unconscious.
Either way, moments later the Trashcan Man arrives, hauling a nuclear warhead out of the desert and into the assembled crowd. The ball of energy summoned by Flagg to attack Whitney transforms into the "hand of God" and detonates the weapon, destroying Las Vegas and vaporizing everyone at ground zero.