Horner was born in the town of Holice, Bohemia, Austria-Hungary (in what is now the Czech Republic), to German-speaking Jewish parents.

Following Max Reinhardt to New York, Harry Horner assisted Reinhardt in his staging of the Biblical musical spectacle The Eternal Road (Der Weg der Verheissung); the production had music by Kurt Weill; conducted by Harry Horner; opening at the Manhattan Opera House 1/7/1937-5/15/1937 with scenic design, costume design and lighting by Norman Bel Geddes.

Max Reinhardt, disgusted with Norman Bel Geddes lack of interest by not showing up for production rehearsals, pressed Harry Horner into acting as art director supervising the sets, costumes, and the lighting.

One of his first notable successes was George Cukor's A Double Life (1947) and he soon found himself up on the Oscar podium in 1949 for his work on William Wyler's The Heiress.

He worked with Cukor again in 1950 on Born Yesterday and then tried his hand at directing on several TV series, including Gunsmoke.

Horner's papers are preserved at the UCLA Charles E. Young Research Library Special Collections and Archives.