In the mid-eighteenth century the music played before an opera—what came to be identified as the overture—was known as a sinfonia. There was no formal distinction between such a work and a work that, despite having the same name, was actually closer to what is now understood as concert symphony. As composers began to more fully explore the possibilities of the latter, however, the difference between the two continued to widen. Such a distinction became evident in Mozart's music: by the time he wrote the comic opera La finta giardiniera ("The Pretend Gardener [-Girl]") for a performance in Munich in 1775, his opera sinfonias were clearly of more modest means than those intended primarily as concert works.

Mozart fully expected the sinfonia from La finta giardiniera to be played as a separate work in its own right, and planned it accordingly. At a later date, however, he combined it with an additional movement (K. 121) to create a multi-sectional (if brief) concert work. The origins of the this appended finale are somewhat unclear; it is not known whether it was written specifically for this purpose or if, like the sinfonia, Mozart intended it as an independent work and later pressed it into service as part of the present work.

The tone of this single-movement Sinfonia is notably light for Mozart's concert symphonies of the same period. The first section is succinct, without a significant development section. The central Andantino grazioso, for strings alone, is likewise quite brief. The final Allegro provides a bright, cheerful finale.

The Sinfonia is scored for relatively modest forces: two oboes, two horns and strings. While the opera was scored for a considerably larger ensemble—adding parts for clarinets, flutes, trumpets, and drums—and a version of the Sinfonia in this scoring is sometimes performed, the manuscript of this instrumentation is not in Mozart's hand and is probably not genuine.

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