After graduating from Princeton University in 1971, Berg expanded his senior thesis on editor Maxwell Perkins into a full-length biography, Max Perkins: Editor of Genius (1978), which won a National Book Award. His second book Goldwyn: A Biography was published in 1989.
Berg was born in Norwalk, Connecticut. The son of Barbara (Freedman) Berg and film producer Dick Berg, Berg was raised Jewish. When Berg was eight, his family relocated to Los Angeles, California. While a sophomore at Palisades Charter High School, Berg researched the author F. Scott Fitzgerald (a favorite of Barbara's, who named her son in part after Fitzgerald) for a report and "developed a mania" for his writing. Berg read all of Fitzgerald's works and later recalled: "It was the first time I saw the fusion of an artist and his life, a tragic and romantic life."
He applied to Princeton University, primarily because it was Fitzgerald's alma mater, and was accepted in 1967. At Princeton, Berg performed in the Princeton Triangle Club theater troupe and considered dropping out to become an actor, though he was convinced by English professor Carlos Baker, a well-regarded biographer of Ernest Hemingway, to "graduate, so at least you'll be an actor with a college degree". Berg studied under Baker, who offered him "constant encouragement and counsel" on his senior thesis, which was a study of editor Maxwell Perkins's career between 1919 and 1929.
After graduating from Princeton in 1971, Berg decided to expand the thesis into a full-length biography, thinking it would take around nine months. He also formulated a career plan at this time, and later recalled: "I did tell myself early on: I think it would be interesting, perhaps, to spend a career writing a half-dozen biographies of twentieth-century American cultural figures—each one, as I often use as my metaphor, a different wedge of the great apple pie." The Perkins biography, Max Perkins: Editor of Genius, took longer than Berg anticipated and was eventually published in 1978, winning a National Book Award in Biography. In 2016, The New Yorker credited Berg with "almost single-handedly rescu[ing] Maxwell Perkins from the anonymity he desired."
In 1982, Berg was approached by Samuel Goldwyn, Jr. to write a biography of his father, the independent film producer Samuel Goldwyn. Berg initially turned the project down, telling Goldwyn that "he was interested in American culture, not Hollywood," but changed his mind after visiting Goldwyn's archives and discovering gin rummy I.O.U.s, menus from Goldwyn's dinner parties, and "all the quotidian minutiae that are a biographer's dream". He won a 1982 Guggenheim Fellowship, which helped finance his work on the biography. The same year, Berg wrote the story for Making Love, a controversial film that was the first major studio drama to address the subjects of homosexual love, closeted marriages, and coming out. He also narrated Directed by William Wyler, a 1986 documentary about the filmmaker William Wyler for which Berg interviewed Wyler, Bette Davis, Audrey Hepburn, Laurence Olivier, and Barbra Streisand, among others. In 1989, Berg published Goldwyn: A Biography, his second biography.
After completing Goldwyn in 1989, Berg began the search for his next subject, who he wanted to be "another great American cultural figure but — because I had written about Perkins and Goldwyn — not somebody from the worlds of publishing or film". After briefly considering Tennessee Williams, Berg decided to research the aviator Charles Lindbergh, attracted by what he described as "the dramatic possibilities of the story of the great hero who became a great victim and a great villain". Berg convinced Lindbergh's widow, Anne Morrow Lindbergh, to grant him unprecedented access to the man's archives, which he was surprised to find totaled "1,300 boxes, or several million papers".
The biography, Lindbergh, was highly anticipated; prior to its publication, the book's film rights were bought, sight unseen, by Steven Spielberg, who planned to direct a movie of it. Published in 1998, Lindbergh sold about 250,000 copies in hardcover, and won the Pulitzer Prize for Biography or Autobiography. Berg was noted for his exhaustive research, as well as his sympathetic, but by no means uncritical, approach to Lindbergh, whose alleged anti-Semitism he addressed in a straightforward, unblinking manner.
From 1998 to 2000, Berg wrote Kate Remembered, a biography-cum-memoir detailing his 20-year friendship with the Hollywood actress Katharine Hepburn. The book was published on July 11, 2003, only 12 days after Hepburn's death. It spent 11 weeks on the New York Times Nonfiction Best Seller list, but received uneasy critical response. In The New York Times, Robert Gottlieb called it an "odd and unsettling book [that leaves] a sense of exploitation", and gossip columnist Liz Smith, a friend of Hepburn's, called Berg "vain and narcissistic", and declared the book "[s]elf-promoting fakery....Hepburn would have despised it and his betrayal of her friendship." Berg responded in a written statement, saying that he was "truly shocked at Liz Smith's professional behaviour — or, more accurately, her lack thereof" in "her personal assault on my reputation, one that stops just short of character assassination".
Berg served on Princeton University's Board of Trustees from 1999 to 2003. In 2000, he began researching a biography of Woodrow Wilson, of whom Berg says, "I have an image of him in my mind that is unlike any picture I have seen anywhere else, based on material at Princeton and 35 years of researching and thinking about him". Wilson was published on September 10, 2013.
In the 2010s, Berg began working increasingly in film and television. He worked for Warner Bros. on a unrealized film adaptation of his favorite childhood television series, 77 Sunset Strip, and served as an executive producer of Genius, the 2016 film adaptation of his Maxwell Perkins biography. Berg was also a consulting producer on the 2017 Amazon series The Last Tycoon.
In 2017, Berg announced that he was researching a biography of Thurgood Marshall, explaining that a definitive biography had not been written and that the project would allow him to explore the subject of race, "the most important topic this country must grapple with in the next few decades".
Berg lives with his partner Kevin McCormick, a film producer, in Los Angeles. His brothers are Jeff Berg, former CEO of International Creative Management, a leading Hollywood talent and literary agency; and music producer and musician Tony Berg. His youngest brother Rick is a partner and manager at the production company Code Entertainment. His niece is Z Berg, a musician of The Like and JJAMZ.