[1] Klein entered the Metropolitan Opera Children's Chorus at age ten, and in 1972 sang two solo parts, Yniold in Pelleas et Melisande and Zweiter Knabe in The Magic Flute, before his voice changed.
[4] Until he returned to opera in his twenties, he had built a career making and playing folk instruments such as banjo, dulcimer, autoharp and bones, a pursuit that he has never completely abandoned.
Although most of his performances there have been in comprimario roles such as Yaryshkinin in The Nose, Chevalier Delaforce in Dialogues of the Carmelites, and Fyodor in War and Peace, he also sang Steva in Jenůfa (2002) and The Witch in Hansel and Gretel (2008).
[9] In 2007, Klein sang the role of Sly in the American premiere of Pascal Dusapin's opera Faustus, the Last Night at the Spoleto Festival USA.
In the Verdi repertoire, he has sung Alfredo, the Duke of Mantua, Manrico, Cassio and Otello, while his Mozart roles include Tamino and Belmonte.