The bill must be approved by the N.C. House and Gov. Mike Easley before becoming law. If it does pass, it would go into effect in December but would not be retroactive.
Sen. Frank Ballance, D-Bertie, the bill's sponsor, said the ban would be contingent on an individual scoring 70 or below on an IQ test, exhibiting "significant subaverage intellectual function."
But the IQ test must have been taken before the murder is committed for the defendant to be exempt from execution, and any defendant who is found to be mentally incompetent still could receive a life-without-parole sentence.
A defendant found to be mentally disabled after committing the crime can introduce evidence from the IQ test but still could receive the death penalty.
Senators opposed to the bill claimed that it would take power from the state's judicial system and give it to the legislature.
But Ballance said the bill is necessary to protect those individuals who commit horrendous crimes "with the mind of a 12-year-old."
Ballance said communities calling for a moratorium on the death penalty played a role in this bill.
"I think it would be folly for any representative not to regard the wishes of his constituents," he said.
Carrboro and Chapel Hill town governments, along with several other N.C. municipalities, have both passed resolutions calling for a moratorium on the death penalty.