The company said the contract worker was shocked shortly before 5 p.m. at its facility in Texas City.
The worker died after he was transported to a medical facility, Marathon said.
"Our top priority is safety at our facilities, and an investigation will be conducted to determine the cause of the accident," Marathon said in a statement. "We extend our deepest condolences to the family and friends of the deceased individual and are providing support to other members of our team during this difficult time."
Marathon did not provide further information about the man who was killed or what led up to the electric shock.