[3] A writer since childhood, Whannell worked as a reporter and film critic for several Australian television shows, including ABC's Recovery, a Saturday morning youth-oriented program that was hosted by Dylan Lewis.

The result was that instead of following the usual MTV ideal of what teenagers want in a TV show—"Hey kids, coming up next we’ve got some seriously WICKED windsurfing moves!

!”—Recovery managed to tap into the so-called "alternative" movement that was in full swing at the time by giving teenagers what they actually want: genuine, unpolished anarchy.

[4] Whannell appeared in Episode 4, Season 1 of the RMITV production Under Melbourne Tonight presents What's Goin' On There?

Saw III grossed $33,610,391[6] on its opening weekend, making around $129,927,001 worldwide (after 38 days in cinemas) and is currently the most successful Saw film to date.

In 2008, Whannell took off his "writing hat" to perform alongside Nathan Phillips in Dying Breed, a low-budget Australian horror film about a team of zoologists exploring the Tasmanian wilderness to locate a creature thought extinct, the thylacine, or Tasmanian tiger.

Instead, they wander into the domain of cannibals who retain their ancestor Alexander Pearce's taste for human flesh, and become prey.

Spending time in a hospital inspired him to endow the lead antagonist of the Saw series, John Kramer / Jigsaw, with cancer.

[7] Whannell wrote the script for and acted in the 2011 paranormal thriller film, Insidious, which was directed by Wan and produced by Oren Peli.

[9]Media reports were published in mid-2013 in regard to Cooties, a film project that Whannell is an executive producer, actor and screenwriter for.

Whannell also added that he doesn't rule out collaborating with Wan again, but he felt like he needs to direct a film for the first time.

Whannell next wrote, directed, and co-executive produced a retooling of H. G. Wells' book The Invisible Man.

In June 2022, he was in talks to direct a reboot film based on the Green Hornet titled The Green Hornet and Kato from Amasia Entertainment and Universal Pictures[16] The success of The Invisible Man led Universal Pictures to team with him again on Wolf Man, which he co-wrote with his wife, Corbett Tuck.