Ln (Ln = La and Ce) atom reactions with methylamine are carried out in a pulsed-laser vaporization supersonic molecular beam source. A series of dehydrogenation species are observed with time-of-flight mass spectrometry, and the dehydrogenated Ln-containing species in the formula Ln(NCH) are characterized by mass-analyzed threshold ionization (MATI) spectroscopy and density functional theory and multiconfiguration spin-orbit coupling computations. The MATI spectrum of La(NCH) consists of two vibronic band systems that are assigned to the ionization of the A ground state of the C isomer La(N-CH) and the A' ground state of the C isomer La(NH-CH). The MATI spectrum of Ce(NCH) also displays two band systems, which are attributed to the ionization of the low-energy spin-orbit coupling states of the C isomer Ce(N-CH). Ln(N-CH) is formed by the concerted dehydrogenation of the amino group, while La(NH-CH) is formed by the dehydrogenation of both amino and methyl groups. Ce(NH-CH) is presumably formed in the reaction based on the computational predictions but not observed by the spectroscopic measurements.