English theatre manager who pioneered the British repertory movement, influencing 20th-century drama, acting, and production.

The heiress of a wealthy tea merchant, Horniman studied at the Slade School of Fine Art (1882–86), but after visiting Germany, where she was impressed by the state-subsidized repertory theatre and by productions of Henrik Ibsen's plays, she decided to make the theatre her lifework. In 1894–95 she secretly subsidized a repertory season at the Avenue Theatre in London, during which Arms and the Man, George Bernard Shaw's first publicly produced play, and William Butler Yeats's Land of Heart's Desire were important landmarks. Thus introduced to the new Irish theatre movement, Horniman spent five years as Yeats's unpaid secretary, during which time she financed the conversion of an old building into the new home of the Abbey Theatre in Dublin (1904).

In 1908 she began her own repertory theatre, the Gaiety, in Manchester. Good plays—from Greek tragedy to works by Shaw, John Galsworthy, Arnold Bennett, Harley Granville-Barker, and St. John Ervine—a first-rate company, and her own managerial talents made Horniman's Gaiety famous. The company toured England and the United States, stimulating the formation of other repertory groups. Financial difficulties beset the company, however, and it was disbanded in 1917; Horniman sold the theatre in 1921. In 1933 she was made a Companion of Honour.