The `Lethal Weapon` star was driving his 2008 Maserati on the Malibu Canyon Road in California at around 8.35pm when the accident occurred but escaped unharmed.
According to website TMZ, a report by the California Highway Patrol read: "For unknown reasons, Mr. Gibson steered his car to the right and struck the rock hillside. He stopped his car at the scene and was contacted by officers from the West Valley CHP Area."
Officer Leland Tang said Mel was alone in the car and no drugs or alcohol were involved, but admitted the actor was baffled as to what had gone on.
He said: "Gibson told officers in the field he did not know how the car drifted out of the lane and into the hillside.
"It was totally an unintentional act."
Although Mel denied being on the phone at the time of the accident, he would not confirm whether or not he had fallen asleep behind the wheel.
Mel cooperated with officers and was given a lift home by a friend after his vehicle was towed away.
The CHP report added: "Mr. Gibson supplied all the pertinent information to the officers at the scene, gave a statement, and received a ride home from a friend."
The crash is the latest controversy in Mel`s life as he is currently being investigated by police following claims he abused ex-girlfriend Oksana Grigorieva, mother of his nine-month-old daughter Lucia.