The first movement is in cut time, and it is the only first movement of a piano concerto by Mozart to be written in cut time. The orchestra opens quietly with a prelude of 71 bars (Hutchings incorrectly states 72), wherein six orchestral themes are exposed (A-F in Hutchings' notation; see main article on Mozart Piano Concertos for a discussion of this notation), of which the first, rhythmical and with a military ambiance, becomes increasingly important as the movement progresses; indeed, its insistent rhythm dominates the entire movement.

The piano then answers with its own exposition of 116 bars, starting with A and B, then introducing some new material (themes x and y), with free passages of arpeggios and scales: the scheme is ABxAyA Free D Free. The orchestra then returns on its own with its short first ritornello (22 bars) that introduces another theme, G: the scheme is AGAG. In the ensuing middle section (35 bars) yet another orchestral theme is introduced, H: the scheme is HAHAHA. This is followed by a long recapitulation, also of 116 bars, where, as is typical of his concertos, Mozart rapidly departs from a simple repetition of the previous material: the scheme is ABAyADA Free. Finally, the movement is brought to a close with the final ritornello (36 bars): AGA Cadenza (Mozart's own exists) EF - hence the two closing themes of the prelude are finally heard again at the end.