The pioneering composer noted for contributions to musical theater in America is George Gershwin. He is famous for blending elements of classical music with jazz to create iconic works such as "Porgy and Bess" and "Rhapsody in Blue." Gershwin's unique style pushed the boundaries of American theater music, contributing greatly to its evolution. His ability to capture the spirit of the times with vibrant melodies and innovative rhythms solidified his status as a foundational figure in both American music and theater.