She made her Broadway debut in New York City in "Sweet Kitty Bellairs" in 1903. Her first leading role was "Fanny Perry" in 1909 in Leo Ditrichstein's "Is Matrimony a Failure?", produced by David Belasco, and then she played stock. This was followed by "The Gamblers" (1910), her first great success, and by "Within the Law" (1912), "Common Clay" (1915), and other successes (New International Encyclopedia). She was known for her interpretation of Shakespearean roles, playing Juliet, Cleopatra, and Viola on Broadway. She made Broadway history by playing "Juliet" over 1000 consecutive performances in 1923; critic George Jean Nathan declared her "not ... the best Juliet that I have seen, but she is by all odds the most charming". Cowl's affecting performances led her to be described as having a "voice with a tear." Biographer Charles Higham admired Cowl's "marvelous bovine eyes and exquisite genteel catch in the voice ..."