Jordan Gelber is the son of Donald and Barbra Gelber. He was born in Riverdale neighborhood of the Bronx, New York City, and grew up in Woodmere, Long Island. Gelber has a younger sister, Marielle. Gelber is Jewish. In 1993, he graduated from George W. Hewlett High School in Hewlett, Long Island.
In 1994, he attended the National Theater Institute.
He is a 1997 graduate of Stanford University, where he was a member of the Stanford Improvisors improvisational theatre troupe and an a cappella group, Stanford Mendicants.
In 2000, Gelber earned a master's degree in acting from New York University's Graduate Acting Program at the Tisch School of the Arts.
He is known for originating the role of struggling Comedian Brian in the Tony Award-winning Broadway musical Avenue Q. Gelber also performed on Broadway in Arthur Miller's All My Sons as Frank Lubey from 2008 to 2009, and he played Buddy in Elf from 2012 to 2013. In 2017, at the Hudson Theatre, Gelber played Louis/Billy in the limited-run Broadway revival of the Stephen Sondheim/James Lapine musical Sunday in the Park With George starring Jake Gyllenhaal and Annaleigh Ashford.
In November 2009, Gelber married Marsha Loeb, a senior online medical Editor at McGraw-Hill. They have two sons, Henry and Dashiell.