Scully worked as a speechwriter in the 2000 presidential campaign, and served as a special assistant and senior speechwriter for President George W. Bush from January 2001 to August 2004. He has also written speeches for vice-presidents Dan Quayle and Dick Cheney, Governor Robert P. Casey of Pennsylvania, and Bob Dole. Most recently, he was the primary consultant and writer for Sarah Palin's acceptance speech at the 2008 Republican convention.

In an article in The Atlantic Monthly, Scully accused former White House chief speechwriter Michael Gerson of wrongfully taking credit for speeches allegedly written by Scully and other members of President George W. Bush's speechwriting team.

Scully is the author of Dominion: The Power of Man, the Suffering of Animals, and the Call to Mercy (2002), described by Natalie Angier in a book review published in The New York Times (October 27, 2002) as a "horrible, wonderful, important book," in large measure "because the author, an avowed conservative Republican and former speechwriter for George W. Bush, is an unexpected defender of the animals against the depredations of profit driven corporations, swaggering, gun-loving hunters, proponents of renewed 'harvesting' of whales and elephants and others who insist that all of nature is humanity's romper room, to play with, rearrange, and plunder at will. Nichols Fox in a review published in the The Washington Post wrote that Dominion is "destined to become a classic defense of mercy." G. Gordon Liddy has also published a positive review, saying "A beautifully written and very powerful book."

Scully has also served as literary editor of National Review magazine, and has written for The New York Times, The Wall Street Journal, The Washington Post, The Los Angeles Times, and The Atlantic Monthly.