DNA methylation (5-methylcytosine, 5mC) plays critical biological functions in mammals and plants as a vital epigenetic marker. The Ten-Eleven translocation dioxygenases (TET1, 2, and 3) have been found to oxidize 5mC to 5-hydroxymethylcytosine (5hmC) and then to 5-formylcytosine (5fC) and 5-carboxylcytosine (5caC) in mammalian cells. We report herein three mushroom TET homologues from Coprinopsis cinerea that can mediate 5mC oxidation. Specifically, one homologue (CC1G_05589, CcTET) shows similar activity to its mammalian TET homologues. Biochemically, CcTET actively converts 5mC to 5hmC, 5fC, and 5caC under natural conditions (pH 7.0). Interestingly, CcTET also converts the majority of 5mC to 5fC under slightly acidic (pH 5.8) and neutral conditions. Kinetics analyses of the oxidation by CcTET under neutral conditions indicate that conversion of 5mC to 5hmC and 5hmC to 5fC are faster than that of 5fC to 5caC, respectively. Our results provide an example of a TET homologue in a non-mammalian organism that exhibits full 5mC-to-5caC oxidation activity and a slight preference to producing 5fC. The preferential accumulation of 5fC in the in vitro oxidation reactions under both neutral and acidic conditions may have biological implications for 5mC oxidation in fungi species.