Charles Logan has been under house arrest since resigning as president after being thwarted by Kiefer Sutherland's Jack Bauer - but is due to return to the show in an episode shown on Sky1 on April 25.
The scheming politician, played by Gregory Itzin, is brought in to consult with President Allison Taylor and will continue through the remainder of the final season of the real-time drama.
"It's pretty nifty to be invited back to this groundbreaking show and be part of how they wrap up the story," Gregory said.
Referring to Logan, Gregory revealed: "I'm a little more of a pragmatist, a realist. President Taylor needs somebody to whisper in her ear and take her to the dark side, because she's such a good person. She's such a straight arrow!"
Gregory, who turns 62 this month, has also appeared on scores of television shows including LA Law, The Practice, Boston Legal and The Mentalist - but the actor said Logan was his biggest role.
"24 put me on the map after years of being a journeyman actor," he said.
He revealed he had been almost begging 24 executive producer Howard Gordon for a return.
"Right about the time that I told myself, 'This borders on begging', Howard said, 'You know, a couple of the writers have been talking...'"
But Gregory remained tight-lipped about how the show will end. "They played around with different endings," he revealed. "One was a Jack-rides-into-the-sunset kind of thing, which made me go, 'Ahhh, that would be nice.' And then they said, 'But we're not going to do that.'"