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Mordden was born and raised in Pennsylvania, in Venice, Italy, and on Long Island, and is a graduate of Friends Academy in Locust Valley, New York, and the University of Pennsylvania. He at first sought a career in show business, working as music director on off-Broadway and in regional theatre, and enrolling in the BMI Lehman Engel Musical Theater Workshop run by Lehman Engel. As both composer and lyricist, Mordden wrote musicals based on William Shakespeare's Measure For Measure and on Max Beerbohm's Zuleika Dobson, but he ended up earning his living as a writer of English prose. In the 1970s, he was assistant editor to Dorothy Woolfolk on such DC Comics as The Dark Mansion of Forbidden Love.
His stories, novels, essays, and non-fiction books cover a wide range of topics including the American musical theater, opera, film, and, especially in his fiction, the emergence and development of contemporary American gay culture as manifested in New York City. He has also written for The New Yorker, including three works of fiction, Critic At Large pieces on Cole Porter, Judy Garland, and the musical Show Boat, and reviews of a biography of the Barrymores and Art Spiegelman's graphic novel Maus. He later became a book reviewer for The Wall Street Journal.
His best known fictional works are the interrelated series of stories known collectively as the "Buddies" cycle. In book form, these began with 1985's I've a Feeling We're not in Kansas Anymore. The fifth in the series, 2005's How's Your Romance?, is subtitled Concluding the "Buddies" Cycle. Together, the stories chronicle the times, loves, and losses of a close-knit group of friends, men who cope with the challenges of growing up and growing older.
Mordden’s 1995 novel How Long Has This Been Going On? follows the lives of a diverse group of men and women from 1949 to 1991 and moving from Los Angeles to the Midwest, then from San Francisco to the Northeast. All but one of the principal characters are gay or lesbian. Mordden's own favorite among his works of fiction is The Venice Adriana (1998), built around the life and art of the opera soprano Maria Callas. Mordden's A Bad Man Is Easy To Find, published in 1989 under the pseudonym of M. J. Verlaine is a series of interrelated short stories about the lives of women, and has only one minor gay character. In 2008, Mordden published The Jewcatcher, a surrealistic novel set in Berlin from the end of the Weimar Republic to the last day of the European war. The many principal characters are a combination of Mordden's inventions and such real-life figures as Adolf Hitler, Marlene Dietrich, Raoul Wallenberg, Claus von Stauffenberg, and President Paul von Hindenburg. In 2012, Mordden published his first volume of gay fiction in eight years, The Passionate Attention of an Interesting Man. In the form of a novella and four short stories, the book explores relationships in which one man dominates another. In 2015, Mordden returned to writing historical fiction with One Day in France, set in Limoges and Oradour-sur-Glane when the latter, a peaceful village, was destroyed and its inhabitants brutally murdered by a squad of the Nazi Schutzstaffel.
Mordden's nonfiction includes seven volumes detailing the history of the Broadway musical from the 1920s through the 1970s (followed by an eighth volume going up to 2003 in a different style from the seven-title series), guides to orchestral music and operatic recordings, and a cultural history of the American 1920s entitled That Jazz. He has also published Demented, an examination of the phenomenon of the operatic diva, and a coffee-table book on the works of Rodgers and Hammerstein. His 2012 book Love Song: The Lives of Kurt Weill and Lotte Lenya is a dual biography chronicling the romance and professional collaboration of Kurt Weill and Lotte Lenya, and in 2013 he published Anything Goes: A History of American Musical Theatre. He has written a number of books on film, including analyses of the influence of Hollywood studios and of the role of female film stars.
- Buddies series
- I've a Feeling We're Not in Kansas Anymore: Tales from Gay Manhattan, 1985
- Buddies, 1986
- Everybody Loves You: Further Adventures in Gay Manhattan, 1988
- Some Men Are Lookers, 1997
- How's Your Romance?, 2005
- The Golden Age of the Broadway Musical
- Make believe : the Broadway musical in the 1920s, 1997
- Sing for your supper : the Broadway musical in the 1930s, 2005
- Beautiful mornin' : the Broadway musical in the 1940s, 1999 
- Coming up roses : the Broadway musical in the 1950s, 1998
- Open a new window : the Broadway musical in the 1960s, 2001
- One more kiss : the Broadway musical in the 1970s, 2003
- The Happiest Corpse I've Ever Seen: The Last Twenty-Five Years of the Broadway Musical, 2004
- All That Glittered: The Golden Age of Drama on Broadway 1919–1959, 2007
- Works on Musical Theatre
- The American Theatre, 1967
- Better Foot Forward : The History of American Musical Theater, 1976
- That Jazz! : An Idiosyncratic Social History of the American Twenties, 1978
- The Hollywood Musical, 1981
- Broadway Babies: The People Who Made the American Musical, 1983
- The Fireside Companion to the Theatre, 1988
- Rodgers & Hammerstein, 1999
- Ziegfeld: The Man Who Invented Show Business, 2008
- On Sondheim: An Opinionated Guide, 2015
- When Broadway Went to Hollywood, 2016
- All That Jazz: The Life and Times of the Musical Chicago, 2018
- On Streisand: An Opinionated Guide, 2019
- Works on Opera
- Opera in the Twentieth Century : Sacred, Profane, Godot, 1978
- A Guide to Opera Recordings, 1980
- A Guide to Orchestral Music, the Handbook for Non-Musicians, 1980
- The Splendid Art of Opera: A Concise History, 1980
- Demented: The World of the Opera Diva, 1984
- Opera Anecdotes (Oxford Paperbacks), 1988
- The New Book of Opera Anecdotes, 2020
- Works on Hollywood
- Movie Star: A Look at the Women Who Made Hollywood, 1983
- The Hollywood Studios, 1988
- Other Published Works
- Smarts, the cultural I.Q. test, 1984
- Pooh's workout book, 1984
- The Jewcatcher, 2008
- The Guest List: How Manhattan Defined American Sophistication---from the Algonquin Round Table to Truman Capote's Ball, 2010
- Love Song: The Lives of Kurt Weill and Lotte Lenya, 2012
- The Passionate Attention of an Interesting Man, 2013
- One Day in France, 2015
- Library of Congress Name Authority
- Gewen, Barry, The New York Times Book Review, December 12, 2004, p. 20.
- Mordden, Ethan (1999). Beautiful Mornin: The Broadway Musical in the 1940s. U.S.: Oxford University Press. ISBN 0-19-512851-6.