The electronic and structural properties of the one-electron-oxidized form of methylcobalamin (MeCbl) cofactor have been investigated using density functional theory (DFT) and CASSCF/MC-XQDPT2 calculations. We applied two types of functionals (hybrid and GGA) which produced quite different results in terms of spin density profiles: the B3LYP description was consistent with Co(III) and the pi-cation corrin radical while the BP86 result was more in line with the Co(IV) oxidation state. A closer inspection of both outcomes indicates that the oxidized species have a mixed pi-cation corrin radical and Co(III)/Co(IV) character. This mixed character was further supported by high-level ab initio CASSCF/MC-XQDPT2 calculations, which reveal the strong mixing of the electronic states due to nondynamical correlation effects. The near degeneracy, which takes place between the ground and first excited state, was consistent with the presence of a pseudo-Jahn-Teller (pJT) effect in the oxidized form of MeCbl. In addition, the DFT-based investigation of the structurally related porphyrin complexes gives a description consistent with corrin-based analogues and reveals that the corrin species have more Co(IV) character. The most important finding of the present study, regardless of the type of functional used, was the significant lowering of dissociation energy (similar to 35%), which might be due to the partial depopulation of the Co-C sigma orbital upon removal of an electron.