As a young adult she was one of the Belmore Sisters in variety entertainment before beginning a more serious acting career performing in classic plays by William Shakespeare with Ben Greet's Pastoral Players in a 1911 tour of the United States.

In more serious work, Belmore portrayed the nurse in the United States premiere of Jean Anouilh's Antigone (1946), and starred as Ftatateeta in the 1949 revival of George Bernard Shaw's Caesar and Cleopatra.

[3] In 1911 Belmore went to the United States as a member of Ben Greet's Pastoral Players, touring the east coast of America in performances of plays by William Shakespeare.

[4] In 1912 she made her Broadway debut at the Lyric Theatre as Portia in Shakespeare's Julius Caesar with William Faversham as Marc Antony and Fuller Mellish in the title role.

[5] In 1920 Belmore made her debut in London's West End at the Empire Theatre as Helen Cheston in Harry Tierney's Irene; a hit production which ran for 399 performances.

[7] She remained active on Broadway for the next couple of years, appearing as Henriette Deschamps in Guy Bolton's Grounds For Divorce (1924-1925) and the Ziegfeld Follies of 1925[8] with W. C. Fields and Will Rogers.

[10] One of the parts she portrayed there was Ethel in Norma Mitchell and Russell Medcraft's Cradle Snatchers; a role which she repeated at the Baltimore Auditorium in 1928 and the Coliseum Theatre in 1931.

[14][15] Her other Broadway appearances included Terence De Marney's and Percy Robinson's The Whispering Gallery (1929, Lady Elliot[16]), Julian F. Thompson's The Warrior's Husband (1932, Forrest Theatre, as Caustica[17]), Jerome Kern and Oscar Hammerstein II's Show Boat (1932, Casino Theatre, Parthy Ann Hawks[18]), Arthur Schwartz's Virginia (1937, Center Theatre, as Minnie Fortesque[19]), Elmer Harris's Johnny Belinda (1940-1941, Mrs. McKee), Lesley Storm's Heart of a City (1942, Mrs. Good[20]), Gypsy Rose Lee's The Naked Genius (1943, Lollie Adams[20]), Fritz Kreisler Rhapsody (1944, Frau Tina Hugenhaugen[21]), Terence Rattigan's Harlequinade (1949, Dame Maud Gosport[20]), and George Bernard Shaw's Caesar and Cleopatra (1949-1950, Ftatateeta[20]).