He is best known for his roles as Ben Lucini in Home and Away, Detective John Grant in Profiler, Cole Turner in Charmed, Christian Troy in Nip/Tuck, Doctor Doom in the Fantastic Four duology, Jonah in Runaways, and Jess LaCroix in CBS crime drama FBI: Most Wanted.

In the early 1990s, McMahon struggled to obtain a work permit for the United States and as a result he missed out on a few roles.

[10] McMahon's big break Hollywood role was on the soap opera Another World when he was cast as Ian Rain in 1993.

[citation needed] In 2018, McMahon returned to Australia to film Swinging Safari in Queensland, alongside Guy Pearce, Kylie Minogue, Radha Mitchell, Asher Keddie, and Jeremy Sims.

[12] McMahon and his ex-sister-in-law Kylie Minogue share an improvised kiss in a scene where couples partner swap.

McMahon departed the series mid-way through the third season; his final episode, "Shattered", aired 8 March 2022.

[15][16] McMahon jokingly noted in an interview with Jimmy Kimmel that his mother [was] "a very scary lady" because she would visit him on the set of his films and critique his performance when he finished a scene.

[18] He starred in her music video clip for the single "This Is It", a song that summed up the couple's relationship and hopes for the future.

[19] The newlyweds spent a great deal of their marriage apart, due to McMahon pursuing his career in Los Angeles and Minogue in England.

[18] In 1999, McMahon married Baywatch star Brooke Burns and had a daughter[20] in June of 2000 before the couple divorced in 2001.