Hi. Mel Gibson was born on Jan 3rd., 1956 Peekskill New York. He moved to Sydney Australian with his family when he was 12 yrs, old. As his Mother is Irish, he has a duel nationality of Irish-American.

Birth name Mel Colm-Cille Gerard Gibson, is an American actor, a film director, producer and screenwriter who was born on January 3, 1956, in Peekskill, New York. He is the sixth of eleven children, and the second son of Hutton Gibson and Anne Reilly. Due to his mother’s Irish nationality, Gibson holds dual Irish and American citizenship. After he got into a lawsuit in a work related injury and being awarded $145,00 against New York Central Railroad, Hutton Gibson planned to relocate with his family to Sydney, Australia, in 1968 when Mel was 12 years old. In Australia he got educated by members of the Congregation of Christian Brothers at St. Leo’s Catholic College in Wahroonga, New South Wales. After Mel completed high school, he went on to study at the University of New South Wales in Sydney where he performed at the National Institute of Dramatic Arts alongside future film thespians, Judy Davis and Geoffrey Rush.

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Source : http://www.biographyarchive.com/biography-of-mel-gibson.html