Alongside No. 20, this concerto is noted as one of Mozart's highly accomplished piano concertos from his peak period. Both were composed for subscription concerts during Lent. Concerto No. 21 premiered on March 10, 1785, approximately one month after No. 20. Mozart, who continued to undertake an enormous amount of work, is said to have completed this piece only on the day before its performance.

In contrast to the D minor of No. 20, Concerto No. 21 possesses a bright sound in C major, exhibiting a clear beauty, especially in its slow movement, akin to celestial music.

Mozart's own cadenzas for the outer movements have not survived.

First Movement: Allegro, C major, 4/4 time. Concerto-sonata form.

The opening motif, which seems to stride triumphantly, characterizes this movement. The solo piano embellishes its simple and clear melodic figures with intricate passages. It is interesting to note the appearance of a motif in the piano part that later appears in Symphony No. 40 in G minor.

Second Movement: Andante, F major, 2/2 time. Ternary form.

This movement is famous for being filled with exceptionally beautiful melodies. These melodies are often taken by the solo piano.

Third Movement: Allegro vivace assai, C major, 2/4 time. Rondo form.

It features many decorative movements typical of a lively finale. The themes are equally distributed between the orchestra and the piano, creating a sense of harmony.