Siegel was born in The Bronx, New York, and received his BA, MA and M.Phil. from Columbia University. He worked as an editor at The New Leader and ARTnews before turning to writing full-time in 1998. The New York Times called him "one of the most eloquent and acid-tongued critics in the country". David Rieff wrote about Siegel that "to read him is to be reminded of what criticism used to aspire to in terms of range, learning, high standards, and good writing and--dare one say it?--values".

In 2002, Siegel received the National Magazine Award for Reviews and Criticism. The citation lauded Siegel's essays as "models of original thinking and passionate writing... tough-minded yet generous criticism is prose of uncommon power--work that dazzles readers by drawing them into the play of ideas and the enjoyment of lively, committed debate." Siegel has written several essays for art catalogues, and several introductions to reprinted classics: D. H. Lawrence's The Lost Girl, Gershom Scholem's Story of a Friendship, and Janet Malcolm's In the Freud Archives. He is the author of several books, including: Falling Upwards: Essays in Defense of the Imagination (2006), Not Remotely Controlled: Notes on Television (2007), and Against the Machine: Being Human in the Age of the Electronic Mob (2008).

Siegel has been the book critic for The Nation, art critic for Slate, television critic for The New Republic, staff writer for Talk magazine, staff writer for Harper's, contributing writer for The Los Angeles Times Book Review, and associate editor of Raritan. Since June 2006, he has served as senior editor at The New Republic.

Siegel is known for coining the term "blogofascism", a term that he meant to describe what he considered the blogosphere's dominant rhetoric of insult, intimidation and attack, which he claimed was exemplified by Daily Kos founder Markos Moulitsas. Siegel is now a visiting professor at Rutgers University.

In September 2006, Siegel was suspended from The New Republic, after an internal investigation determined he was participating in misleading comments in the magazine's "Talkback" section, in response to anonymous attackers on his blog at The New Republic's website. The comments were made through the device of a "sock puppet" dubbed "sprezzatura", who, as one reader noted, was a consistently vigorous defender of Siegel, and who specifically denied being Siegel when challenged by an anonymous detractor in "Talkback". In response to readers who had criticized Siegel's negative comments about TV talk show host Jon Stewart, 'sprezzatura' wrote, "Siegel is brave, brilliant, and wittier than Stewart will ever be. Take that, you bunch of immature, abusive sheep." The New Republic posted an apology and shut down Siegel's blog. In an interview with the New York Times Magazine, Siegel dismissed the incident as a "prank". He resumed writing for The New Republic in April 2007. Siegel's critique of Web culture, Against the Machine: Being Human in the Age of the Electronic Mob, was published in January 2008.

Siegel lives in New Jersey with his wife and son.