The Harvard Law professor emeritus took issue with Mueller stating earlier in the day that "if we had had confidence that the president clearly did not commit a crime, we would have said so" in his report.
"That was absolutely inappropriate for him to say. It was worse than anything Comey said when he exonerated Hillary Clinton and then said, but she engaged in extremely careless conduct," he said.
Dershowitz said Comey was broadly "condemned" for his July 2016 announcement about the Clinton email probe and "helping the Democrats" was Mueller's only possible motive" for his appearance Wednesday morning.
Dershowitz continued that Mueller's decision to emphasize the possibility that President Trump committed a crime "went well beyond his authority as special counsel."
"A special counsel was a terrible, terrible mistake and I think Mueller's statement today proves that beyond any doubt," he said.
Dershowitz repeated his long-held belief that a "nonpartisan investigative commission" should have been tasked with examining Russian election interference in 2016 and recommending steps to combat future interference.
In an op-ed for The Hill, Dershowitz wrote that "no prosecutor should ever say or do anything for the purpose of helping one party or the other."
"I cannot imagine a plausible reason why Mueller went beyond his report and gratuitously suggested that President Trump might be guilty, except to help Democrats in Congress and to encourage impeachment talk and action. Shame on Mueller for abusing his position of trust and for allowing himself to be used for such partisan advantage," he stated.