Gadon started acting at the age of 10 with her first acting role as Julia in an episode of La Femme Nikita (1998). For the next few years, she took episodic roles in various television series, including Monica in Are You Afraid of the Dark? (1999), Young Laura Burnham in Twice in a Lifetime (2000), Catherine Hartman in Mutant X (2002), Claudia in Dark Oracle in 2004, Vicki in Life with Derek (2005), and Tasha Redford in Flashpoint (2008).

She also has a number of television films to her credit. She was nominated for the Young Artist Award for Best Ensemble in a TV movie for her first film, The Other Me (2000), portraying Heather. Other roles include Samantha in What Girls Learn (2001), Amanda in Cadet Kelly (2002), Julia Norton in Code Breakers (2005) and Celeste Mercier in The Cutting Edge: Chasing the Dream (2008).Gadon had recurring roles in many television series: Zoe Kessler in The Border (2008–2009), Katie Atkins in Being Erica (2009), Georgia Bravin in Happy Town (2010) and Ruby Odgen in Murdoch Mysteries (2009–11). She is also behind the voice of the title character in the animated series Ruby Gloom (2006–07), Beth in Total Drama (2007–2011) and Portia in Friends and Heroes (2007–09). Gadon was nominated for a Gemini Award in 2008 for Best Individual or Ensemble Performance in an Animated Program or Series for her work in Ruby Gloom (2008).

Her filmography includes both feature length and short films. Her first feature film was Fast Food High (2003) where she portrayed Zoe. She portrayed Margaret in the dark comedy Siblings, Priscilla in Charlie Bartlett (2007) and Laura in Leslie, My Name is Evil (2009).

In 2005, she filmed for Where Love Reigns, a promotional film co-starring Douglas Henshall.

Her short film work includes Haley in Burgeon and Fade (2007), Julia in Grange Avenue (2008) and Gabrielle in Spoliation (2008). Burgeon and Fade won the Special Jury Award at the WorldFest Houston Festival for original dramatic short film. She also starred in the short indie film, The Origin of Teddy Bears, as Madison.

In 2011, Gadon starred in David Cronenberg's Cosmopolis, alongside Robert Pattinson, as his on-screen wife of 22 days, Elise Shifirin. She played Phillippa in the television adaptation of Ken Follett’s bestseller World Without End, an eight-hour event series. She featured in Brandon Cronenberg's debut feature, Antiviral as Hannah Geist, a mega-star in a sci-fi world where fans pay to be infected with the diseases of the rich and famous.

In 2012, she appeared as Carl Jung's wife Emma in the David Cronenberg film A Dangerous Method and in a Canadian short film, Yellow Fish, alongside J. Adam Brown. On May 23, 2012 in Cannes, Birks presented the first Birks Canadian Diamond award to Gadon and Emily Hampshire during Telefilm Canada's inaugural Tribute To Canadian Talent press event and reception.

She played Miss Elizabeth Murray in the 2013 release of the film Belle. She co-starred in Denis Villeneuve's Enemy (2013), based on the José Saramago book, The Double (2002), and in David Cronenberg's Maps to the Stars (2014), a dark comic look at Hollywood excess.

She participated in the CBC "Canada Reads" competition in March 2014. In September 2014, it was announced that she was cast in Miramax's supernatural thriller The 9th Life of Louis Drax, along with Jamie Dornan and Aaron Paul. The film was released in September 2016. Gadon played Dracula's wife Mirena (and briefly the modern-day Mina) in the historical action film Dracula Untold, released in October 2014.

Gadon made her directorial debut with an episode of Reelside, a documentary series, which focused on her collaborative relationship with Photographer Caitlin Cronenberg; the episode premiered on The Movie Network in Canada June 4, 2015. In 2015, Gadon appeared as Princess Elizabeth in A Royal Night Out, a heavily fictionalized account of the Future Queen's incognito night on the town, along with her sister Princess Margaret, on the evening of VE Day.

In 2016, Gadon starred opposite Logan Lerman in Indignation, an adaptation of Philip Roth's 2008 novel of the same title, and opposite James Franco in 11.22.63, a television mini-series version of Stephen King's novel of the same title.