Thiessen grew up in Upper East Side in Manhattan, where both his parents were "liberal Democrat" doctors. His mother grew up in Poland and fought as a teenager in the Warsaw Uprising, a military struggle in which his grandfather died.

Thiessen is a graduate of the Taft School (1985), a private prep school in Connecticut. He graduated from Vassar College (BA in 1989) and completed post-graduate studies at the Naval War College.

He lives in Alexandria, Virginia with his wife Pamela, who is currently the Staff Director of the Senate Republican Policy Committee, and their four children.

Thiessen has worked in Washington for many years, starting with five years at Black, Manafort, Stone and Kelly. He spent six years (1995—2001) on Capitol Hill as spokesman and senior policy advisor to Senate Foreign Relations Committee Chairman Jesse Helms (R-NC).He joined the Bush administration as Chief Speechwriter for Donald Rumsfeld in 2001, then moved to Bush's speechwriting team in 2004. In February 2008, he became chief speechwriter when William McGurn resigned.

In March 2009, Thiessen and Peter Schweizer opened Oval Office Writers LLC.

Thiessen is currently a visiting fellow at the Hoover Institution. He is also a columnist for The Washington Post, starting March 2010.

Thiessen's first book, Courting Disaster: How the CIA Kept America Safe and How Barack Obama Is Inviting the Next Attack (ISBN 1596986034), was published by Regnery Publishing in January 2010. The book is endorsed by the former Vice President Dick Cheney, former Secretary of Defense Donald Rumsfeld, and former Attorney General Michael Mukasey. It reached the No. 9 spot on the New York Times Best Sellers list for hardcover nonfiction in February 2010.

Jane Mayer, author of The Inside Story of How The War on Terror Turned into a War on American Ideals, a book which Thiessen says has fundamental errors of fact, heavily criticized Courting Disaster in a book review, claiming it is "based on a series of slipshod premises." In a long response, Thiessen defended the accuracy of his book and said Mayer's review contained many factual errors and omissions. For example, Mayer quoted the head of Scotland Yard’s anti-terrorism branch in 2006 as saying the book's account of the Heathrow plot is "completely and utterly wrong"; in reply Thiessen quoted a former senior CIA official as saying that the CIA liaises only with MI6 and MI5, so the Scotland Yard official "would have no way of knowing what intelligence the CIA shared with MI6 or MI5, much less the ultimate source of that intelligence". Thiessen added, "The week her article appeared in The New Yorker, former CIA director Mike Hayden handed it out in his class at George Mason University’s School of Public Policy as an example of all that is wrong with intelligence journalism today." (Hayden had earlier written a positive review of the book for The Daily Caller.

Matthew Alexander, a former military interrogator, and author of How to Break a Terrorist, characterizes Thiessen's book as "a literary defense of war criminals".

Mark Oppenheimer wrote in the New York Times "Beliefs" column that many Catholic thinkers strongly disagree with Thiessen's assertion in the book that waterboarding is permitted by the teachings of the Roman Catholic Church.

Thiessen's promotional tour for Courting Disaster included a confrontational interview with CNN's Christiane Amanpour and an interview with Jon Stewart on The Daily Show, an uncut version of which was posted online.