Gallagher's work on the New York stages spanned seven decades, with her big break coming in the role of Nancy in the 1947 musical High Button Shoes.

Gallagher won her first Tony Award and a Donaldson Award for her role as Gladys Bumps in the 1952 revival of Pal Joey, and earned her first leading role on the Broadway stage in 1953, starring in Hazel Flagg.

She played Maeve for the show's duration, from July 1975 to January 1989, and was recognized with three Daytime Emmy Awards.

She appeared in Make a Wish, Hazel Flagg, Portofino, High Button Shoes, and Sweet Charity (for which she received a 1967 Tony Award nomination for Featured Actress in a Musical), eventually assuming the title role, and closing the original Broadway run.

Her first starring role on Broadway came in 1953 as title character in Hazel Flagg, based on the 1937 Carole Lombard movie Nothing Sacred.

[7] Despite extensive work on Broadway, Gallagher is perhaps best known to many Americans unfamiliar with her theater repertoire as the Irish matriarch Maeve Ryan on the ABC soap opera Ryan's Hope, a role she played for the show's entire duration, from 1975 to 1989.

Creator and head writer Claire Labine scripted the end of the final episode with Maeve at the family bar, singing "Danny Boy".

Almost immediately after the cancellation of Ryan's Hope, Gallagher had a two-day guest stint on Another World, and has appeared in All My Children as a strict nurse and on One Life to Live as a sex therapist.