Early in her career, (when she was still known as Merle Letowt), she played "Thelma" in the original cast of Gypsy (1959). Eventually, she moved up to the fourth lead rôle of "Dainty June", playing it for much of the Broadway run and on the first national tour.
Louise returned to Broadway in Company (1970) as "Susan". Her biggest success was as "the Beggar Woman" in Sweeney Todd (1979), for which she won the Drama Desk Award as Outstanding Featured Actress in a Musical. However, her "ravaging crazy" performance is not preserved in the video recording as she was not a member of the national tour that was videotaped.
In Into the Woods (1987), she played a trio of rôles: the ethereal "Cinderella's Mother", the practical "Granny" (of "Little Red Riding Hood" fame), and the unseen vengeful "Giantess".
Louise has originated rôles with other Broadway composers as well. She played "Mme. Dindon" in Jerry Herman's La Cage aux Folles (1983) and "Molina's Mother" in Kander and Ebb's Kiss of the Spider Woman (1993), a rôle she also played on London's West End, in Toronto and on the national tour.
Off-Broadway, she created the rôle of Cecily MacIntosh in Charlotte Sweet (1982). Regionally, she is an acclaimed actress in classics by Shakespeare, Chekhov, Molière, Pinter, Ibsen, and Shaw.
The cast of the Broadway show, title of show (along with some of their theatrical friends), created a game based around Merle Louise, entitled "Six Degrees of Merle Louise", which is based on Six Degrees of Kevin Bacon except that it generally uses Broadway musicals in place of movies to link two performers.
Louise's on-camera work has been limited, most notably appearing in the televised production of Into the Woods and a documentary about the recording of the Original Cast Album of Company. She has had some guest appearances on shows such as Law & Order.
Her most recent performances include "Frau Schneider" in Cabaret, "Mme. Armfeldt" in A Little Night Music and "Jeanette" in The Full Monty in Massachusetts, and in New York she appeared in Luke Yankee's award-winning show, The Jesus Hickey in July 2007.Billy Elliot: The Musical (Grandma understudy/Ensemble) 2008
Kiss of the Spider Woman (Molina's Mother) 1993
Into the Woods (Cinderella's Mother/Granny/Giant)1987
La Cage aux Folles (Mme. Dindon) 1983
Sweeney Todd (Beggar Woman) 1979
Company (Susan) 1970
Gypsy (Thelma; later Dainty June) 1959