We demonstrate electro-optic ultrafast control of the optical quality factor of an on-chip silicon microcavity. The micrometer-sized cavity is formed by light confinement between two microring resonators acting as frequency selective mirrors. The ring resonators are integrated into p-i-n junctions enabling ultrafast injection and extraction of carriers. We show tuning of the cavity quality factor from 20,000 to 6,000 in under 100 ps. We demonstrate both high-Q to low-Q and low-Q to high-Q transitions.