A jury convicted 25-year-old Christian Carney Tuesday in the death of 20-year-old Stephen Serrano.
In 2008, Serrano was driven to a secluded part of San Clemente and was shot in the face and back.
Carney faces up to 28 years and eight months to life in prison.
His sentencing is scheduled for next month.
Another Marine, 24-year-old Alvin Lovely, is still awaiting trial in the killing.