Loudon made her Broadway debut in the 1962 musical Nowhere to Go But Up. The show lasted only a week, but Loudon received good reviews and a Theatre World Award for her performance.
In 1969, Loudon appeared in The Fig Leaves Are Falling, about a middle-aged man leaving his wife and family for a younger woman. The cast included her co-star Barry Nelson and David Cassidy, of "The Partridge Family" fame, who made his Broadway debut in the production.
Though this musical was also short-lived, playing just four performances, Loudon received her first Tony Award nomination for her portrayal of Lillian Stone.
Loudon achieved great success when she starred as Miss Hannigan in the original Annie, which opened on Apr. 21, 1977, at the Alvin Theatre (now the Neil Simon Theatre). She won the Tony Award for Best Actress in a Musical that year, beating out 13-year-old co-star Andrea McArdle, who played the title role.
Critics loved the show and praised Loudon's performance. In his New York Times review, Clive Barnes wrote: "As the wicked Miss Hannigan, Dorothy Loudon, eyes bulging with envy, face sagging with hatred, is deliciously and deliriously horrid. She never puts a sneer, a leer or even a scream in the wrong place, and her singing has just the right brassy bounce to it."
The following year, Loudon starred opposite Vincent Gardenia in the musical Ballroom. She earned her third Tony Award nomination for her portrayal of Bea Asher, a lonely widow who takes up ballroom dancing and becomes romantically involved with a mailman she meets at the local dance hall. Loudon famously performed the stand-out song "Fifty Percent."
Directed and choreographed by Michael Bennett, the production opened on Dec. 14, 1978, at the Majestic Theatre and ran for 116 performances.
The production opened on Nov. 19, 1981, at the Ethel Barrymore Theatre and marked Hepburn's final Broadway apppearance.
The production ran for 553 performances at the Brooks Atkinson Theatre and received a Tony Award nomination for Best Play.
Loudon's subsequent Broadway performances were in the 1985 musical revue Jerry's Girls, celebrating the songs of composer-lyricist Jerry Herman, and 1994's Comedy Tonight, a variety show featuring four comedians.
Her final Broadway appearance was in the 2002 Broadway revival of Dinner at Eight. She performed the role of Carlotta Vance in the show's first preview, then dropped out of the production due to illness. Marian Seldes stepped in for the ailing Loudon.