A fourth-generation actress on her father's side, Gillmore trained at the American Academy of Dramatic Arts.

Her stage acting career stretched from The Scrap of Paper in 1917 through to Noël Coward's musical Sail Away on Broadway in 1961.

[1] She was first noticed by the critics in the 1919 play The Famous Mrs. Fair, in which she appeared with Henry Miller and Blanche Bates.

In 1921 she played the tubercular patient Eileen Carmody in Eugene O'Neill's The Straw, and in 1922 she portrayed Consuelo in the United States premiere of Leonid Andreyev's He Who Gets Slapped.

During World War II, Gillmore had a role in the traveling production of The Barretts of Wimpole Street.