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Born and raised in Brooklyn, New York, Mandelbaum was introduced to Broadway musical theatre by his parents and grandparents at an early age. He initially pursued an acting career, studying with Stella Adler and performing at Circle in the Square and the Provincetown Playhouse.
In 1986, he began writing for Show Music Magazine  and the New York Native, and the following year he joined the staff of TheaterWeek. He was a frequent contributor to Playbill and wrote a regular column for Broadway.com until 2006. Prior to his career as a theatre writer, he was a teacher in New York public schools.
Both books are regarded to be the definitive discussion of their respective topics by theatre historians and musical theatre buffs. Kirkus Reviews called A Chorus Line a "Must-read history of musical theater." The New York Times called Not Since Carrie a "must read...Mr. Mandelbaum hits all the highlights, everything from Breakfast at Tiffany's, starring Mary Tyler Moore as Holly Golightly, to last season's Shogun: The Musical. Mr. Mandelbaum also takes a good swipe at Annie 2: Miss Hannigan's Revenge...
- Jones, Kenneth. "Show Music Magazine Expected to Cease Publication After Fall 2002 Issue" playbill, September 16, 2002
- Viagas, Robert. "TheaterWeek Ceases Publication" playbill, January 7, 1997
- "Kirkus Review" kirkusreviews.com, July 24, 1989, accessed July 30, 2019
- Not Since Carrie macmillan.com, accessed July 30, 2019
- Wichtel, Alex. "On Stage, and Off" The New York Times, September 20, 1991