Julian Dana William McMahon was born in Sydney, Australia, the second of three children of Lady Sonia McMahon (née Sonia Rachel Hopkins) and Sir Billy McMahon, the longest continuously serving government minister in Australian history, serving over 21 years as a government minister before serving as Prime Minister of Australia from March 1971 to December 1972. Sir Billy died March 31, 1988, age 80, four months before Julian's 20th birthday, and Julian's mother, Lady (Sonia) McMahon, died of cancer,