From February 1784, Mozart began keeping a catalog of his works to manage them. This was created by Mozart himself to quickly retrieve works for concerts or publication, and it records the title, date, instrumentation, and incipit on the grand staff.

According to this catalog, the Sonata in C minor is dated October 14, 1784, while the Fantasia in C minor, K. 475, published simultaneously by Artaria, is dated May 20, 1785.

Furthermore, the autograph manuscript, long thought to be lost, was discovered in 1990, revealing many differences in notes compared to the first edition. This suggests that Mozart's autograph manuscripts were primarily for his own performances, and that subtle embellishments, especially in slow movements, were often improvised.

First Movement: C minor, 4/4 time, Sonata form

It begins resolutely with the principal theme, which plays an arpeggiated tonic chord in three-octave unison. After a transitional passage with a chromatically descending inner voice (m. 9 ff.), it reaches the relative major, E-flat major, where the subordinate theme appears (m. 23 ff.), ornamented with circling figures and modified by chromatic alterations. Subsequently, another subordinate theme appears in a dialogue-like manner between the upper and lower voices. This leads to a transitional passage featuring many chromatic passages and motives from the principal theme. The first half, which would normally close in the key of the subordinate theme, remains open in this sonata, ending on a dominant ninth chord of the tonic key.

The open ending of the first half serves as a connection, on one hand, to the beginning via the repeat sign, and on the other hand, to the second half. The second half (m. 75 ff.) begins with the principal theme in C major, followed by the first subordinate theme (m. 79 ff.) and the principal theme (m. 83 ff.) in F minor. The principal theme then modulates through G minor and C minor, accompanied by eighth-note triplets, reaching a dominant ninth chord and pausing on a dominant seventh chord, similar to the ending of the first half. At measure 99, marked with a long fermata, it would be possible to insert an Eingang (short cadenza) according to the performance practice of the time.

After the recapitulation of the principal theme (m. 100 ff.), the recapitulation of the first subordinate theme is omitted, and motives from the principal theme are developed polyphonically (m. 118 ff.). The second subordinate theme is then recapitulated in the tonic key (m. 131 ff.). Furthermore, a coda based on the principal theme is provided (m. 168 ff.), closing the movement pianissimo.

Second Movement: E-flat major, 4/4 time

This is a typical slow movement in Mozart's keyboard works, where the theme is varied each time it returns. The dominant theme (m. 8 ff.) contains many chromatic embellishments.

Following the first return of the theme (m. 17 ff.), a new musical idea appears in the subdominant key of A-flat major (m. 24 ff.). After rapid ascending and descending scales and arpeggios, the theme returns in a more embellished form (m. 41 ff.).

Third Movement: C minor, 3/4 time

The rondo theme, in a rapid Allegro assai (agitato) tempo, features a syncopated rhythm connected by an anacrusis and ties, creating a melody that sounds like a hemiola shifted by one beat, supported by metrically regular chords.

The passage (m. 24 ff.) that pauses on a dominant seventh chord, sustains tension through a general pause, and then proceeds to a double dominant, possesses a tension that seems to anticipate 19th-century Romantic harmony (such as in Wagner's Tristan und Isolde).

Chromatic passages are also found in the couplet theme in the relative major, E-flat major. After the return of the rondo theme (m. 103 ff.), the couplet theme appears in the tonic key (m. 167 ff.).

The return of the rondo theme from measure 221 includes an inserted section with stacked dominant seventh and dominant ninth chords, marked a piacere. This increase in harmonic tension can be considered one of the characteristics of Mozart's minor-key works. In the coda, the range is extensively expanded, using almost the entire range of the instrument to conclude the piece.