Raised as a Quaker in the picturesque Cotswold town of Burford, Warner decided to forego a university education, opting instead to study stage management at drama school. At the age of 21, she started her own theater troupe, Kick, a group of inexperienced young actors who received good notices for their efforts, but were unable to perform frequently due to a lack of funds. When she was just 28, she was invited to join the Royal Shakespeare Company (RSC) as one of that organization's few female directors. She won Olivier and London Evening Standard awards for one of her first RSC outings, a production of the bard's goriest play "Titus Andronicus" (1987). Regarded as something of a wunderkind, the brown-eyed blonde staged numerous interpretations of classics at the RSC and the Royal National Theater, where she became an associate director.

After six years, Warner was persuaded to try her hand at opera by Nicholas Payne, then-director of Opera North and now of the English National Opera. Her first effort was Berg's "Wozzeck" (1993), which received good notices. Her second stab at opera did not fare as well, however. Upon stepping before the curtain at the end of a performance of her "Don Giovanni", Warner was met with hisses and boos from the audience, prompting Gilles Cachemaille, the opera's title character to quip: "I couldn't bear to be in a production of 'Don Giovanni' that wasn't booed. I would think it had broken new ground." Warner was not shaken, however, and defended the audience's right to respond vocally. Unaffected by their reaction, she went on to stage other operas and musical performances, including "Jeanne d'Arc au bucher" for BBC Proms, "The Turns of the Screw" for the Royal Opera House and Bach's "St. John Passion" for the English National Opera.

Warner made her NYC stage directing debut in 1996 with a critically acclaimed, experimental staging of T.S. Eliot's 433-line poem "The Waste Land". for which Warner and Shaw nabbed Drama Desk Awards (Most Outstanding Theatrical Event and Best Solo Performance respectively).

Although Warner had filmed a version of "The Waste Land" for the BBC in 1995 and "Richard II" in 1997, she did not make her feature directorial debut until 1999 with an adaptation of the Elizabeth Bowen novel "The Last September". A beautifully lensed, character-driven movie that garnered respectable notices, this first big-screen outing featured a remarkable cast that included stage legends Maggie Smith and Michael Gambon at the head of an Anglo-Irish family circa 1920, David Tennant and Keeley Hawes as the younger generation around whom the action revolves and Shaw as a glamorous, but aging vamp intent on seizing her last chance of marriage. Following the release of the movie, Warner returned to the stage to direct Shaw in the tragedy "Medea" in Dublin in 2000. Always the innovator, Warner replaced the play's Greek chorus with one that spoke English and Gaelic on alternate nights.