On My Mind is written in the key of G Minor.
According to the Theorytab database, it is the 6th most popular key among Minor keys and the 14th most popular among all keys. Minor keys, along with major keys, are a common choice for popular music. The three most important chords, built off the 1st, 4th and 5th scale degrees are all minor chords (G minor, C minor, and D minor).
See the G Minor Cheat Sheet for popular chords, chord progressions, downloadable midi files and more!