Malloy said as a young man, he was a supporter of capital punishment, but after years working as a prosecutor, he saw too many defendants poorly served by their counsel, wrongly accused, mistakenly identified or discriminated against.
"In the trenches of a criminal courtroom, I learned firsthand that our system of justice is very imperfect," he said in a statement. "In bearing witness to those things, I came to believe that doing away with the death penalty was the only way to ensure it would not be unfairly imposed."
Amnesty International USA released a statement Wednesday praising Malloy for his decision to sign the bill.
"Lawmakers in Connecticut finally saw the death penalty for what it is -- a barbaric and irreversible punishment that does nothing to stop crime or support its victims," said Suzanne Nossel, executive director, Amnesty International USA.
Connecticut joins 16 other states that have banned capital punishment.