Fundamental modelocking to generate short terahertz (THz) pulses and THz frequency combs from semiconductor lasers has become a routine affair using quantum cascade lasers (QCLs) as a gain medium. However, no demonstrations of harmonic modelocking have been shown in THz QCLs, where multiple pulses per round trip are generated when the laser is modulated at harmonics of the cavity’s fundamental round trip frequency. Here, using time resolved THz techniques, we show for the first time harmonic injection, and active and passive mode-locking where THz QCLs are modulated at harmonics of the round-trip frequency. Furthermore, using the unique ultrafast nature of our approach, we show that passive or self-starting harmonic modelocking originates from the QCL self-generating a harmonic microwave modulation. The latter auto-modulates the gain and loss in the system, spontaneously forcing the QCL to operate up to its 15th harmonic and opening up prospects of passive THz short pulse generation.