Leamer was born in Chicago and later moved to upstate New York with his family where he attended Vestal Central High School. He attended Antioch College where he spent a year in France, studied at the Université de Besançon, and worked in a factory. He received his B.A. in History from Antioch in 1964. After his graduation, Leamer joined the Peace Corps and was stationed in the mountains of Nepal. When he returned home, Leamer studied both at the University of Oregon and the prestigious Columbia University School of Journalism, where he graduated second in his class in 1969 while being named a Pulitzer International Fellow.

After finishing at Columbia, Leamer worked as an associate editor at Newsweek before turning to writing magazine articles for a range of publications, including Harper's, The New York Times Magazine, and Playboy. During this period, Leamer worked in a coal mine in West Virginia, while researching an article. During the Indo-Pakistani War of 1971, he was the only journalist to live in a Bengali hotel in Dacca and travel to remote areas of the newborn Bangladesh. His article in Harper's won a citation from the Overseas Press Club for "Best Magazine Reporting".

Leamer is widely regarded as an expert on the Kennedy family due to his exhaustive trilogy on the family, of which The Kennedy Women was the main selection of the Book of the Month Club and reached number two on the New York Times Best Seller list.. His other two books on the Kennedys, The Kennedy Men and Sons of Camelot were equally well received. He has often been interviewed by NBC Nightly News, The New York Times, CNN and NPR to lend his expertise to matters concerning the Kennedys, Ronald Reagan, and American politics. In the period following the death of John F. Kennedy, Jr., Leamer served as the on-air consultant for MSNBC's coverage of the plane crash and subsequent funeral. He again served as a consultant during the coverage of former President Ronald Reagan's funeral.

Leamer has not limited himself to covering only the Kennedys, as several of his other books have graced best-seller lists, including his biography of Johnny Carson, King of the Night, which spent over 6 weeks on the New York Times Best Seller list. In writing his novel, Assignment, Leamer lived in Peru for two years to research the cocaine trade. He again immersed himself in his topic when he moved to Nashville to research the business and lifestyle of country music and its many stars for Three Chords and the Truth. Each of the books were lauded for the depth of their research. Leamer also briefly flirted with movie fame when his work on the life of famed mountaineer Willi Unsoeld was purchased by Robert Redford's production company to be turned into a film. The film remains in the development stages. Leamer's latest book, Madness Under the Royal Palms details the intrigue of life in Palm Beach among its wealthy inhabitants.

Leamer is married to Vesna Obradovic, who assists him with the research for his books. The author has one daughter, Daniela, and two grandchildren. The couple maintain residences in Palm Beach, Florida and Washington, D.C. Leamer's brother, Edward, is an economics professor at UCLA, the Chauncey J. Medberry Chair in Management, and director of the UCLA Anderson Forecast. His other brother, Robert, is an attorney and executive vice president of a leading hospital group in New York City.