Shaw made it a forum for considering moral, political and economic issues, possibly his most lasting and important contribution to dramatic art.

As Shaw's experience and popularity increased, his plays and prefaces became more voluble about reforms he advocated, without diminishing their success as entertainments. Such works, including Caesar and Cleopatra (1898), Man and Superman (1903), Major Barbara (1905) and The Doctor's Dilemma (1906), display Shaw's matured views, for he was approaching 50 when he wrote them. From 1904 to 1907, several of his plays had their London premieres in notable productions at the Court Theatre, managed by Harley Granville-Barker and J. E. Vedrenne. The first of his new plays to be performed at the Court Theatre, John Bull's Other Island (1904), while not especially popular today, made his reputation in London when King Edward VII laughed so hard during a command performance that he broke his chair.

By the 1910s, Shaw was a well-established playwright. New works such as Fanny's First Play (1911) and Pygmalion (1912), had long runs in front of large London audiences. Shaw had permitted a musical adaptation of Arms and the Man (1894) called The Chocolate Soldier (1908), but he had a low opinion of German operetta. He insisted that none of his dialogue be used, and that all the character names be changed, although the operetta actually follows Shaw's plot quite closely, in particular preserving its anti-war message. The work proved very popular and would have made Shaw rich had he not waived his royalties, but he detested it and for the rest of his life forbade musicalization of his work. Several of his plays formed the basis of musicals after his death—most famously the musical My Fair Lady —it is officially adapted from the screenplay of the film version of Pygmalion rather than the original stage play (keeping the film's ending).

Shaw's outlook was changed by World War I, which he uncompromisingly opposed despite incurring outrage from the public as well as from many friends. His first full-length piece, presented after the War, written mostly during it, was Heartbreak House (1919).

In 1921, Shaw completed Back to Methuselah. The massive, five-play work starts in the Garden of Eden and ends thousands of years in the future;

He wrote plays for the rest of his life, but very few of them are as notable—or as often revived—as his earlier work. The Apple Cart (1929) was probably his most popular work of this era. Later full-length plays like Too True to Be Good (1931), On the Rocks (1933), The Millionairess (1935), and Geneva (1938) have been seen as marking a decline. His last significant play, In Good King Charles Golden Days has, according to St. John Ervine, passages that are equal to Shaw's major works.