Gehman attended McCaskey High School in Lancaster and worked on several daily newspapers in Lancaster before joining the U.S. Army Corps of Engineers in World War II, serving four years as a writer for The Oak Ridge Times in Oak Ridge, Tennessee. After the war he moved to New York City and began writing for Esquire, Life, Time, Cosmopolitan, Colliers, Argosy, True, Saga and Good Housekeeping magazines and was an original Contributing Editor at Playboy, before going freelance. Maurice Zolotow once claimed that Gehman wrote an entire issue of Cosmopolitan using more than a dozen different pen names. Mark Evanier describes Gehman as "a prominent author of his day, specializing in celebrity profiles. He often got access to follow stars around for a few weeks so he could interview them extensively and report on what he observed..."

Gehman was married five times; his third wife was Academy Award Winning actress Estelle Parsons from 1953 to 1958. He fathered at least forty children. Credited with creating the phrase the "Rat Pack," Gehman was considered a "shadow" member of the group. Among Gehman's fifteen nonfiction books are:

• Day of the Locust (introduction to Modern Library Edition) (1950)
• Sardi's: The Story of a Famous Restaurant (1953)
• A Murder in Paradise (1954)
• Frank Sinatra and his Rat Pack (1961)
• The Tall American: The Story of Gary Cooper (1963)
• That Kid: The Story of Jerry Lewis (1964)
• Bogart: An Intimate Biography (1965)
• A Hell of a Life with Harry Richman (1966)
• The Haphazard Gourmet (1966)
• The Sausage Book
• The Jazz Encyclopedia (with Eddie Condon)
• Let My Heart be Broken
• How to Write and Sell Magazine Articles (1959)
• The Best From Cosmopolitan (editor) (1961)
• In The Soup, In A Stew (unpublished)
• Playboy's Playboy: An Intimate Biography of Hugh Hefner (unpublished)

Fiction:

• A Party at the Buchanan Club
• Each Life to Live
• The Slander of Witches (1955)
• Driven (1960)
• The Had (1966)

Play:

• By Hex (1956)

Gehman also contributed the introduction to the Modern Library edition of Nathaniel West's Day of the Locust. He appeared as himself in the Jerry Lewis movie The Patsy.

Gehman taught writing at:

• The Writers' Workshop at the University of Iowa
• New York University
• Columbia University
• Indiana University
• Pennsylvania State University
• Bread Loaf Writers' Conference at Middlebury College.

In the early 1960s Gehman was hired by TV Guide magazine, for which he wrote many articles focused on celebrities. Gehman believed that creative people were often emotionally insecure because of an unhappy childhood, and that those who became celebrities in the entertainment industry sometimes did so because their insecurity motivated them to succeed.