Julian McMahon is the third son of Sir William McMahon, the former Australian Prime Minister, and his wife Sonia, Lady McMahon. He was educated at Sydney Grammar School, a private boys' school. He briefly studied Law at the University of Sydney and Economics at the University of Wollongong, but this bored him and he started a successful modeling career. Because of this, he became known in the fashion capitals Milan, New York, Rome, and Paris.

In addition to a career as a model, McMahon began an acting career at an early age in Home and Away. He appeared in the Dannii Minogue music video for, 'This is It'. His first role in American television was on the soap opera Another World, playing "Ian Rain" from 1993 to 1995.

Nominated for the Golden Globe award for "Best Actor in a Television Series - Drama" in 2005 for his role as Christian Troy in Nip/Tuck. He also played "Dr. Michael Walsh" in the movie Magenta in 1996, "Mick Dooley" in "Wet and Wild Summer!" aka "Exchange Lifeguards" in 1992, "Sheriff Hayes" in "In Quiet Night" aka "You Belong to Me Forever" in 1998, "George Simian" in the movie "Chasing Sleep" in 2000, "David Cameron" in the made for TV movie "Another World" in 2001 and the supervillain Doctor Doom in Fantastic Four, the 2005 film adaptation of the comic book series of the same name.

From 1996-2000 he starred in all four seasons of the NBC drama Profiler as Agent John Grant. Julian also played Cole Turner on the WB's supernatural show Charmed.

He has been married and divorced twice, first briefly to Australian singer Dannii Minogue and subsequently to American actress Brooke Burns, with whom he has a daughter named Madison Elizabeth McMahon born in June 2000.