For whom was atonality an important compositional technique?

Atonality, also know as serialism (and the 12-tone row), was an extremely important compositional technique for Austrian-American composer Arnold Schoenberg (spelled Schönberg in German). So much so, that he is considered to have been the composer and teacher with the most significant contributions during the 1900s. Specifically, with regards to developing and spreading atonality to other musicians and composers. He lived from 1874 to 1951.

Ais the correct one.