The photochemistry and photophysics of the amino-oxo (AO) and imino-oxo (IO) tautomers of 1-methylcytosine are investigated with ab initio calculations, using the CASPT2//CASSCF approach. Our aim is to rationalize recent experimental results which show that the AO tautomer has an excited state lifetime of about 1 ps, similar to cytosine (J.-W. Ho, H.-C. Yen, W.-K. Chou, C.-N. Weng, L.-H. Cheng, H.-Q. Shi, S.-H. Lai and P.-Y. Cheng, J. Phys. Chem. A, 2011, 115, 8406–8418), and that irradiation with wavelengths shorter than 308 nm induces the AO→IO tautomerization (I. Reva, M. J. Nowak, L. Lapinski and R. Fausto, J. Phys. Chem. B, 2012, 116, 5703–5710). For the canonical AO tautomer we find two analogous decay mechanisms to those described previously for cytosine, involving a conical intersection of ethylenic type and one where the amino group is bent out of plane. Decay through these intersections provides an unreactive return path to the AO ground state species. More importantly, we have identified new decay paths that lead from the two intersections to the trans-IO tautomer, without a barrier. These paths provide the possibility of forming the IO tautomer, presumably in small yields, as a side product of the radiationless decay. Thus, we have established for the first time computationally the mechanism of the UV-induced tautomerization, which is compatible with the well-established decay mechanism for cytosine. For the IO tautomer, we also find a mechanism for the excited state interconversion of the cis and trans forms and for efficient radiationless decay through a conical intersection where the imino group is perpendicular to the ring and bent out of the plane. These results are likely to be valid also for cytosine.