The show opened on Oct. 14, 1961, at the 46th Street Theatre (now the Richard Rodgers Theatre). It won seven Tony Awards, including Best Musical, and received the 1962 Pulitzer Prize for Drama.
In 1968, McKechnie appeared in the original production of Burt Bacharach, Hal David and Neil Simon's Promises, Promises. She played Vivien Della Hoya opposite Jerry Orbach as Chuck Baxter and Jill O'Hara as Fran Kubelik.
The musical opened on Dec. 1, 1968, at the Shubert Theatre and enjoyed a 1,281-performance run. In the show's positive New York Times review, Clive Barnes wrote that McKechnie "led the dance number at the end of the first act with the power and drive of a steam hammer in heat."
Two years later, McKechnie originated the role of Kathy in the Stephen Sondheim-George Furth musical Company, about the pleasures and pains of love and marriage in New York City. The opening night cast included Elaine Stritch and Dean Jones, who was replaced early in the run by his standby Larry Kert.
The musical was nominated for a record-breaking 14 Tony Awards, winning six. Kert was nominated for Best Actor in a Musical, marking the only time a replacement actor has been nominated in a competitive category.
McKechnie's next career highlight came in the first Broadway revival of Betty Comden, Adolph Green and Leonard Bernstein's On the Town. She performed the role of Ivy Smith in a cast that included Bernadette Peters as Hildy Esterhazy and Phyllis Newman as Claire DeLoone.
The musical opened on Oct. 31, 1971, at the Imperial Theatre and ran for 73 performances.
Four years later, McKechnie memorably created the role of Cassie in A Chorus Line, which opened on Jul. 25, 1975, at the Shubert Theatre. The opening night cast included Kelly Bishop as Sheila, Pamela Blair as Val, Priscilla Lopez as Diana and Robert LuPone as Zach.
The production won nine Tony Awards, including Best Actress in a Musical for McKechnie, and was awarded the 1976 Pulitzer Prize for Drama. It ran for 6,137 performances and was the longest-running show in Broadway history until it was surpassed by Cats in 1997.
In 1996, McKechnie appeared in State Fair, the stage adaptation of Rodgers and Hammerstein's 1945 musical film of the same name. She starred as Emily Arden alongside John Davidson, Kathryn Crosby and Andrea McArdle.
The production opened on Mar. 27, 1996, at the Music Box Theatre and ran for 110 performances.