Mason’s father was composer Lowell Mason, a leading figure in American church music. His younger brother, Henry Mason, was a co-founder of the piano manufacturers Mason and Hamlin.
After a successful debut at the Boston Academy of Music, William went to Europe in 1849; there he was the first American piano student of Franz Liszt and Ignaz Moscheles. Mason was the leader of a chamber ensemble based in New York that introduced many works of Robert Schumann and other famous Europeans to Americans during the Civil War era and beyond, at a time when classical music still had little specifically American identity. He published numerous pedagogical works for the piano student but is remembered above all for his Chopinesque compositions for piano.
The American composer and pianist Edward MacDowell (1860-1908) dedicated his second piano sonata, Op. 50 Sonata Eroica (1895), to William Mason.