This composer is known almost exclusively for his brief association, during his student years, with Wolfgang Amadeus Mozart. Franz Xaver Süssmayr sometimes spelled his last name "Süssmayer" and sometimes used an Italian version, Francesco Saverio Dolcevillico. His father, Franz Karl Süssmayr, was a teacher and choirmaster in Schwanenstadt, and gave the boy his early lessons in singing, violin, and organ. In 1779 Franz Xaver entered the Kremsmünster monastery school as a boarder. He went on to study philosophy and law at the Ritterakademie in Kremsmünster in 1784-1787. He also found private composition teachers and joined in musical performances in the monastery. He wrote some compositions that were performed there.
In 1788 he went to Vienna and set himself up as a private music teacher. Looking to study composition in greater depth, he enrolled as a private student with Mozart, who also hired him as an assistant. When Mozart died in December of 1791, Süssmayr continued his musical studies with Antonio Salieri, who mainly taught him details of scoring for voices. From 1792 Süssmayr found work as a harpsichordist and acting Kapellmeister of Vienna¹s National Theater.
During the last decade of his short life (he lived to be only two years older than Mozart did) he became popular as a composer of Singspiele in the tradition of Mozart's Die Zauberflöte (The Magic Flute). The most popular was Der Spiegel von Arkadien (1794). Both Beethoven and Paganini borrowed Süssmayr themes as the subjects of variations. His church music was sung well into the nineteenth century in southern Germany and Austria.
However, his reputation survives almost exclusively on account of his completion of Mozart¹s final work, the Requiem K. 626. Süssmayr and Mozart¹s widow left conflicting versions as to how much of it each of the two composers wrote. The general opinion among scholars is that Mozart left portions of the score in draft form, with vocal parts and harmonies only. Süssmayr, in this majority opinion, realized the instrumentation to this section, and then went on to compose the Sanctus, Benedictus, and Agnus Dei. He retained in his sections of the score the same sorts of thematic links Mozart had used, showing that he had a deep understanding of Mozart¹s structural thinking about the work. It is also very likely that he composed the recitatives in Mozart¹s late coronation opera, La Clemenza di Tito.