The composer who introduced the Mannheim galant style to Vienna, Georg Matthias Monn enjoyed a high reputation in Austria during his lifetime, although his music was not widely circulated beyond German-speaking territories and was generally ignored during the first 250 years following his death.
His baptismal name was actually Johann Georg Mann; he seems to have called himself Georg Matthias to avoid confusion with his younger brother, Johann Christoph, also a composer. "Monn" conforms to the Lower Austrian pronunciation of the family name. J.G. Monn was a child chorister, and in his early twenties he took a job as organist at the new Karlskirche in Vienna. He may also have played the organ at Melk Abbey, although this has been disputed, and it's possible, though not certain, that he was an early teacher of Albrechtsberger.
He did, with Wagenseil, rise to become one of the leading Viennese composers of the mid eighteenth century. His music was performed at the court of Emperor Joseph II, but none of it -- and there seems to be quite a bit of it -- was published during his lifetime. He is noted in the music history books as the first to compose a four-movement symphony with a third-movement minuet, which would become the standard pattern in the works of Haydn and Mozart. He did this exactly once, though, and otherwise held to the traditional three-movement pattern.
The Monn music that is being revived today tends to involve the orchestra; the sinfonia and the concerto represent him in the catalogs. At the same time, this music's textures tend to be thin, more akin to chamber music than orchestral scores. These works, especially the keyboard concertos, are cheerful and highly engaging, with a striking harmonic restlessness that looks ahead to the style of C.P.E. Bach. What limits its popularity, though, is its very conservative structural nature; the concertos favor the old Baroque ritornello format, the chamber music and solo keyboard works are multi-movement partitas, and he was always fond of canonic textures, especially in minor-mode movements. Another limitation is its formulaic thematic cells. Monn tended to write little motifs rather than full-fledged melodies; this facilitated the music's development, but limited its memorability. Some of Monn's scores attracted the attention of an unlikely later composer: Arnold Schoenberg. The father of dodecaphony expanded the continuo part of one of Monn's cello concertos, and later transformed one of the keyboard concertos into a cello concerto for Pablo Casals.