Joni Mitchell has recently been critically received and marketed as a ‘classic’ songwriter. Yet the existing literature has hardly begun to explore the sophistication of her technical achievement. One of her most distinctive contributions is in the realm of harmonic innovation. Through a comprehensive survey of her work, I have found that Mitchell's songs fall under five categories of harmonic organisation: modal, polymodal, chromatic, polytonal, and pedal point. I examine these categories in detail, choosing a variety of examples from the early years of her career (1966–1972). My analyses are intended to contribute to a more knowledgeable appreciation of Mitchell's intellectual stature as a composer. While many songwriters have been inventive within traditional tonal harmony, Mitchell's work is impressive for its extended exploration of alternatives to single key structures and the major/minor system.