Van Brett Watkins also pleaded guilty to conspiracy to commit first-degree murder, said assistant district attorney Gentry Caudill.
Watkins admitted in court that he fired the gun that killed Cheirca Adams, Caudill said.
"We anticipate that he will testify in the other cases," the prosecutor said.
Watkins, Carruth, Michael Eugene Kennedy and Stanley Drew "Boss" Abraham all were charged with first-degree murder in the shooting of Adams.
Adams was shot four times from a passing vehicle as she drove her car in a Charlotte suburb on Nov. 16.
Adams was pregnant with Carruth's child at the time. The boy was delivered by emergency Caesarean section hours after the shooting. Adams died Dec. 14.
Watkins' plea came just two weeks after reports that prosecutors had called off any plea bargains that would have allowed the defendants to plead guilty to second-degree murder.
It also came the same day that a judge held a hearing to determine how much child support Carruth should pay for Chancellor Adams.
Carruth's criminal defense attorney, David Rudolf, who was with Carruth at the child support hearing, said he had not heard anything about Watkins' plea.
"That's irrelevant to us," he said, adding that Carruth maintains his innocence.
At the child support hearing, psychologist Jonathan Gould argued that Carruth should be allowed to visit with his son.
Gould's report was admitted into evidence, then immediately sealed by Judge Yvonne Mims Evans. The hearing is to continue Tuesday.
Adams' parents, Saundra Adams and Jeffrey Moonie, have sued Carruth for custody of Chancellor and for child support. Saundra Adams has temporary custody.
Carruth is being held without bond in the Mecklenburg County jail.