As a teenager he worked at breaking wild horses to saddle—which led to a teen-age career as a stunt man, stand-in, and extra in well-nigh 100 films".

Kert's first professional credit was as a member of a theatrical troupe called the "Bill Norvas and the Upstarts" in the 1950 Broadway revue Tickets, Please!.

[4] After a seven-month run, he worked sporadically in Broadway,[5] Off-Broadway and ballet productions as a dancer until 1957, when he was cast in West Side Story.

A few months later, while he was working for Esquire in an advertising show, Stephen Sondheim approached him after seeing him perform and set up an audition for the part of Tony.

[citation needed] According to Arthur Laurents, who wrote the book for West Side Story, Kert was "a California extrovert, laughing, bubbling, deadly funny, and openly gay.

He was a member of the cast of the infamous ill-fated musical version of Truman Capote's novella, Breakfast at Tiffany's, which closed during previews in December 1966.

In 1998, when Sony Music, which had acquired the Columbia catalog, released a new digital version of the original Broadway cast recording, Kert's rendition of "Being Alive", the show's final number, was included as a bonus track.

One of Kert's last recordings was the 1987 2-CD studio cast album of the complete scores of two George and Ira Gershwin musicals: Of Thee I Sing and its sequel Let 'Em Eat Cake.

[8] His television credits included guest appearances on The Sorcerer's Apprentice (Alfred Hitchcock Presents), Kraft Suspense Theatre, The Bell Telephone Hour, Combat!

Kert's last stage appearance came in a touring company of La Cage aux Folles but he missed performances because of illness.