A man who spent 45 years in prison in Virginia for the sexual assault and killing of a 3-year-old boy was exonerated on Tuesday. Lawyers said he had served more time behind bars than any other client exonerated with the help of the Innocence Project, a legal effort that has freed hundreds of wrongly convicted people across the country.

The state Court of Appeals said that new evidence, including DNA testing and research into false confessions, had dismantled the state’s original case against the man, Marvin Leon Grimm Jr., who had confessed to the 1975 murder of the boy.

The court granted Mr. Grimm a writ of actual innocence, meaning that he is fully absolved of the crime.

The boy, identified in court papers as C.H., was seen entering the woods behind his family’s apartment in Richmond, Va., before he disappeared. His body was later found on the banks of the James River.

In 1976, Mr. Grimm pleaded guilty to abduction, forcible sodomy and murder and was sentenced to two life sentences and 10 years.

Much of the key evidence that linked Mr. Grimm, a neighbor, to the killing was the work of Mary Jane Burton, a senior analyst at Virginia’s crime lab who has since died. Her work was previously linked to wrongful convictions, and is now under a broader review after a podcast raised allegations that she had taken shortcuts and might have even falsified results.

Peter Neufeld, a founder of the Innocence Project, said that Mr. Grimm had pleaded guilty in exchange for an agreement from prosecutors that they would not to seek the death penalty. Because of that, Mr. Neufeld said the Innocence Project had to lobby to change Virginia law in order to allow Mr. Grimm’s request to test DNA evidence.

Among other things, the testing revealed that a substance found in the boy’s mouth that Ms. Burton had identified as semen contained no sperm.

This is a developing story. Check back for updates.