Competition between electron localization and delocalization in Mott insulators underpins the physics of strongly correlated electron systems. Photoexcitation, which redistributes charge, can control this many-body process on the ultrafast timescale. So far, time-resolved studies have been carried out in solids in which other degrees of freedom, such as lattice, spin or orbital excitations, dominate. However, the underlying quantum dynamics of 'bare' electronic excitations has remained out of reach. Quantum many-body dynamics are observed only in the controlled environment of optical lattices where the dynamics are slower and lattice excitations are absent. By using nearly single-cycle near-infrared pulses, we have measured coherent electronic excitations in the organic salt ET-F2TCNQ, a prototypical one-dimensional Mott insulator. After photoexcitation, a new resonance appears, which oscillates at 25 THz. Time-dependent simulations of the Mott-Hubbard Hamiltonian reproduce the oscillations, showing that electronic delocalization occurs through quantum interference between bound and ionized holon-doublon pairs.