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, three days after the 22nd anniversary of her husband's passing, in Sydney, on April 2, 2010, age 77, with Julian and his two sisters at her bedside.
Julian is of Irish and English descent. Julian started a Bachelor of Arts degree at the University of Wollongong, but after more time spent in the University bar than at classes, he became bored after one year and began a career in modeling, working primarily in commercials. In 1987, he began print modeling assignments in Los Angeles, New York, Milan, Rome and Paris. His appearance in a TV commercial promoting jeans in his home country made him popular enough to be cast as the lead in The Power, the Passion (1989), an Australian "Dynasty"-like series. After 18 months on "The Power, The Passion," Julian then joined the cast of Home and Away (1988), another successful Australian series, where he won a best actor award from a national magazine.
McMahon later performed on stage, appearing in a musical version of "Home and Away" in Britain as well as in "Love Letters" in Sydney and Melbourne. After a lead role in the feature film Wet and Wild Summer! (1992) with Elliott Gould, he moved to Hollywood so that he could read for more American projects. In 1992, he was cast as Ian Rain on NBC's daytime drama Another World (1964). He left "Another World" after two years, in order to expand his range and experience, appearing in several Los Angeles stage productions. He also appeared in the feature film Magenta (1997) before landing the role of Agent John Grant on Profiler (1996) for four seasons,
In his free time, McMahon enjoys surfing, biking, and cooking. He is a fan of baseball, American football and basketball, and he collects classic books.