Wenham started his career as an actor after graduating from Theatre Nepean at the University of Western Sydney with a Bachelor of Arts (Performing Arts) in 1987. Wenham's television credits include several telemovies, such as his AFI award-winning role in the 1996 telemovie Simone de Beauvoir's Babies; and his role as the outwardly laid back but deeply enigmatic diver Dan Della Bosca in the 1998 and 1999 seasons of the highly successful ABC television series SeaChange. His role as "Diver Dan" has made the actor something of a sex symbol, although he dislikes thinking of himself as such, and he has been voted Australia's "sexiest man alive". A portrait of Wenham by Artist Adam Cullen won the Archibald Prize in 2000. Wenham is signed to Storm Model Management in London.
Australian films Wenham has starred in include the critically acclaimed The Boys (1998) based on the play of the same name premiered at Griffin Theatre Company and in turn based on the murder of Anita Cobby; Molokai (1999), based on the life of Father Damien; The Bank (2001); Gettin' Square (2003); Stiff (2004); The Brush Off (2004) and Three Dollars (2005). Wenham has periodically appeared in Hollywood films; he is known for playing Faramir, son of Denethor II, in New Line Cinema's The Lord of the Rings: The Two Towers and The Lord of the Rings: The Return of the King.
He was also seen in Van Helsing playing Hugh Jackman's sidekick, Friar Carl. His character, Dilios, narrated and appeared in the movie 300. Minor roles of Wenham's in overseas films include in The Crocodile Hunter as a park ranger, and briefly in Moulin Rouge! as Audrey. Wenham stars in the music video for Alex Lloyd's single "Brand New Day". In 2008's Australia, he reunited with Hugh Jackman playing antagonist Neil Fletcher who seeks to acquire the ranch Jackman's character is employed with.
In 2009, he again took to the stage, this time as the lead actor, Jerry Springer, in the British musical Jerry Springer: The Opera. During its 6-day run at the Sydney Opera House he played in sold-out performances alongside ARIA award-winning singer Kate Miller-Heidke.
In 2010, Wenham starred as the disgraced Melbourne Lawyer Andrew Fraser in the Australian TV series Killing Time. This ten-part series shows Fraser's fall from grace as he defends many Melbourne Criminals during the 1980s and 1990s. It was shown on TV1 in late 2011.
In 2013, Wenham returned to the stage to play the lead role of John Proctor, in the Melbourne Theatre Company's mid-year production of Arthur Miller's The Crucible.
In 2014, Wenham starred as Patrick Jones in Paper Planes, released on 15 January 2015. That same year, Wenham voiced the role Jacko a frilled-neck lizard, in Blinky Bill the Movie.
Wenham played the role of Harold Meachum in the Netflix original TV series Iron Fist, which premiered in March 2017.
