The phenothiazines have been in use as antipsychotics for over half a century. Although it has been well-documented that these drugs also exhibit anti-cancer activity, a conclusive mechanistic explanation for how this occurs has remained elusive. We report here that these drugs inhibit histone acetyltransferase activity. A preliminary screen of a yeast mutant library indicated that many of the HDAC mutants were resistant to a classic antipsychotic, chlorpromazine. Further investigation revealed that treating glioblastoma cells with this drug reduced histone acetylation. To determine whether this activity was a general characteristic of this group of drugs, thioridazine and trifluoperazine, two members of the other classes of phenothiazines, were investigated in HAT activity assays, and also shown to be HAT inhibitors. Co-treatment with sodium butyrate, an HDAC inhibitor, alleviated the detrimental effect on cell viability in the short term, confirming that the HAT inhibitory activity contributed to the cytotoxicity in cancer cells. These findings indicate the significant potential of these drugs to be repurposed as cancer chemotherapeutics, because their pharmacokinetics are well characterized, and while histone deacetylase inhibitors are increasingly being trialed as anti-cancer treatments, the phenothiazines could provide an important complement based on their HAT inhibitory activity. This would be particularly relevant for cancers such as glioblastoma, which have very poor prognoses, but which are accessible by the phenothiazines.