McMahon was born in Sydney, the son of Sir William McMahon, a prominent Australian politician, and Lady McMahon (née Sonia Hopkins), a socialite and fashion icon. His father was a Member of the Australian House of Representatives and a high-ranking minister in the 24-year Liberal Government for many years before Julian's birth; he became the 20th Prime Minister of Australia in March 1971, when Julian was two years old. His mother left their three children in the care of a nanny to be with her husband in Canberra during his Prime Ministership. He has an older sister, Melinda, and a younger sister, Deborah. He is of Irish descent through his father.