Giovanni Di Stefano, living in Italy, said he filed an application with the Inter-American Commission on Human Rights on Manson's behalf, saying his client didn't receive a fair trial 40 years ago because, among other things, he was not allowed to represent himself, CNN reported Tuesday.
The prosecutor in the Manson case said Di Stefano's claim lacks merit. Manson was assigned a public defender after the judge became bothered with Manson's behavior while representing himself.
Di Stefano said the judge's decision violated Manson's Sixth Amendment rights and is grounds for a new trial.
In a telephone interview with CNN, De Stefano said he took the case because he believes in justice, even though the murders of actress Sharon Tate and six others Manson was convicted of masterminding were "horrendous."
"This is a question of law," Di Stefano said. "I have no interest in the facts of this case. The law is the law."
Prosecutor Vincent Bugliosi said he thought Di Stefano is "wasting his time."
The 1969 murders were memorialized in Bugliosi's book, "Helter Skelter."
Manson, imprisoned at Corcoran State Prison in California, originally was sentenced to death, but the sentence was changed to life in prison when California temporarily abolished the death penalty in 1972.