We investigate the UV vibronic spectrum and excited-state nonradiative processes of jet-cooled 5-methylcytosine (5MCyt) using two-color resonant two-photon ionization spectroscopy at 0.3 and 0.05 cm(-1) resolution. In contrast to cytosine, which shows only five bands above its electronic origin, the lowest electronic transition of 5MCyt exhibits about 25 low-frequency vibronic bands that extend to 0(0)(0) + 450 cm(-1), allowing to extract detailed information on the excited-state electronic and nuclear structure. Most bands are overtones and combinations of the out-of-plane vibrations nu'(1), nu'(2), and nu'(3). Their large intensities reflect butterfly-, boat-, and twist-deformations of the 5MCyt framework upon electronic excitation. From the rotational contours of the 0(0)(0), 1(0)(2), 2(0)(2), and 3(2)(0) bands, the transition is found to be polarized along the in-plane a/b axes, characteristic of a (1)pi pi* transition. Approximate second-order coupled-cluster (CC2) and time-dependent B3LYP calculations both predict that 5MCyt undergoes an out-of-plane deformation in its (1)pi pi* (S-2) state but both methods overestimate the out-of-plane nu'(1), nu'(2), and nu'(3) vibrational frequencies by a factor of 3-5. The TD-B3LYP (1)pi pi* transition dipole moment direction is 10%:90% a:b, in good agreement with experiment. From the Lorentzian line shape contributions needed to fit the rotational contours, a lower limit to the 5MCyt (1)pi pi* state lifetime at the 0(0)(0), 1(0)(2), 2(0)(2), and 3(0)(2) bands is determined as tau >= 30 ps. These values are in stark contrast to the ultrafast (picosecond) lifetimes measured for jet-cooled cytosine by femtosecond pump-probe techniques. They also confirm the observation from the R2PI spectrum that 5-methylation of cytosine increases its excited-state lifetime. The higher out-of-plane overtone and combination bands disappear from the spectrum by similar to 460 cm(-1), signaling the onset of lifetimes tau < 0.5 ps, induced by excitation of the nu'(1), nu'(2), and nu'(3) modes. Above 460 cm(-1) the in-plane fimdamentals 98, 108, and 178 are weakly observed, implying that the in-plane vibrations couple weakly to the out-of-plane modes and have lifetimes T > 3 ps up to 720 cm(-1) excess vibrational energy.