In October 1777, on his way to Paris, Mozart stopped at Stein's instrument workshop in Augsburg and encountered a fortepiano of exceptionally high quality. This is believed to have significantly influenced Mozart's composition of (forte)piano sonatas.

After passing through Augsburg, Mozart arrived in Mannheim in late October and deepened his interactions with the court musicians there. He became particularly close with Christian Cannabich, and in a letter dated November 4th to his father, he mentioned composing a sonata for Cannabich's daughter, Rosina.

However, there is no definitive evidence that this sonata is indeed the one composed for Rosina. Mozart sent this sonata to his father in Salzburg, and it is certain that it was copied there. Since no other Mozart sonata autograph score shows the folds that would have occurred during mailing, it is certainly possible that this sonata, whose autograph is lost, is the one in question. However, recent research suggests that this sonata was composed between November and December, and there is also a theory that the Sonata in D major, K. 311, said to have been composed between October and November, is the one referred to.

In any case, K. 309 and K. 311 were composed in Mannheim around the same time. Stylistic similarities are observed in both sonatas, supporting this claim.

First Movement: C major, 4/4, Sonata Form

It features a powerful opening reminiscent of an orchestral tutti, followed by a softly flowing melody as the main theme, and a light secondary theme in the dominant key (from m. 35). It is a common practice for Mozart that the latter half (development section) begins with the main theme in the parallel minor of the dominant. Using motives from the main theme and the coda as material, the movement proceeds through D minor and A minor before the main theme reappears in the tonic (from m. 94). After the secondary theme reappears in the tonic (from m. 129), the opening motive reappears in the coda, concluding the movement.

Second Movement: F major, 3/4

A set of variations based on a theme characterized by dotted rhythms. In the first variation (from m. 17), it is ornamented with appoggiaturas and 32nd notes. Following an episode in the dominant key (from m. 33), the second variation (from m. 45) features an inversion of the theme's dotted rhythm, along with decorative variations using scale passages. The melody is also octave-doubled, adding brilliance, but the latter half is omitted, leading to a variation of the episodic theme in the dominant (from m. 53). This is followed by a variation driven by dotted rhythms with trills and a bass line of 16th-note triplets, leading to the final theme variation (from m. 65), which incorporates even more virtuosic passages.

Third Movement: C major, 2/4, Rondo

A rondo with a light theme supported by simple arpeggiated harmonies. However, its form is remarkably free, as if it emerged from an improvisation. The return of the rondo theme immediately connects to passages of 16th-note triplets and a couplet theme against a tremolo background on the dominant. It seems that the emphasis is placed more on the display of improvisatory passages and the development of various motives rather than on the rondo theme itself.