Mass-analyzed threshold ionization (MATI) spectroscopy was used to measure the vibronic spectra of LnO (Ln = La and Ce). Single-reference coupled cluster and relativistic multireference configuration calculations were carried out to compare with the measured vibronic energies. The spectrum of LaO displays a single vibronic band system, while that of CeO shows multiple ones. The ionization energies of LaO and CeO are measured as 5.2446(6) and 5.3332(6) eV, respectively, which are a 100-fold improvement over the literature values. The vibrational energies of the neutral molecule and corresponding ion reveal the charge effect on the metal-oxygen bond of both species. The single band system in the spectrum of LaO arises from the transition of the ground state of the neutral molecule with the La(6s)O(2p) valence configuration to the ground state of the singly charged ion with the La(6s)O(2p) configuration. The multiple band systems in the spectrum of CeO are attributed to the spin-orbit coupling for the Ce(4f6s)O(2p) configuration of the neutral molecule and an excited state for the Ce(4f)O(2p) configuration of the ion.