Frank Thornton (1845 – 18 December 1918) was an English actor, singer, comedian and producer. Despite a successful stage career in comedies in London, on tour and abroad, Thornton is probably best remembered as the understudy to George Grossmith in a series of Gilbert and Sullivan operas from 1877 to 1884.
Thornton began his stage career giving drawing-room entertainments while simultaneously working in a commercial office in London, keeping his theatrical activities secret from his office employers for four years. He was engaged by Richard D'Oyly Carte as understudy to George Grossmith in the Gilbert and Sullivan operas, also playing roles in some of the curtain raisers played with the operas. In 1881, he created a small principal role in Patience and resigned from his office post. In 1883, he played the Lord Chancellor in a tour of Iolanthe.
Thornton left the D'Oyly Carte Opera Company in 1884 and began a long series of tours of Australia in stage comedies, notably The Private Secretary, interspersed with appearances in burlesques, farces and other plays on the London stage and on tour. He rejoined D'Oyly Carte in the 1890s in London and on tour, and later returned to touring Australia in comedies including Charley's Aunt.