While at ABT, he originated rôles in Jerome Robbins' Fancy Free and Interplay, in addition to performing in ballets by George Balanchine, David Lichine, Léonide Massine and Antony Tudor.

He made his Broadway debut in the short-lived Mr. Strauss Goes to Boston (1945), then had more success as a soloist in Three to Make Ready (1946) and Look, Ma, I'm Dancin'!

Lang's first major role, however, was as Bill Calhoun/Lucentio in the original production of Kiss Me, Kate (1948) — although he did not always get along with composer Cole Porter.

Lang also performed "Tom, Dick or Harry" (as Lucentio) with Edwin Clay (as Gremio), Charles Wood (as Hortensio) and Lisa Kirk (as Bianca).

It was reported 20th Century-Fox wanted him for the role of Vera-Ellen's character's boyfriend Mike in Three Little Girls in Blue (1946) but he had to refuse because of a stage commitment in Three to Make Ready (1946).