Researchers discovered the work at the city's music library while compiling the latest edition of the so-called Koechel catalog, the definitive archive of Mozart's musical works.
The piece is referred to as "Ganz kleine Nachtmusik" in the new Koechel catalog, according to the Leipzig libraries.
The Koechel catalog describes the piece as "preserved in a single source, in which the attribution of the author suggests that the work was written before Mozart's first trip to Italy", according to the municipal libraries. (This would mean he was 12 or 13 when he wrote this)
A list by Mozart's father had alerted academics to the existence of "many other chamber music compositions" by the young artist, which were all thought to have been lost until the emergence of the string trio, Leisinger said.
"Since the inspiration for this apparently came from Mozart's sister, it is tempting to imagine that she kept the work as a memento of her brother," Leisinger said.
The piece was performed by a string trio at the unveiling of the new Koechel catalog in the Austrian city of Salzburg on Thursday. It will receive its German premiere at the Leipzig Opera on Saturday.
Here's a cleaner version, starts at 1:25:48