Clemens was born in Brisbane, Australia. The daughter of an English father, she lived as a child in Japan, France and Hong Kong, where she attended the Hong Kong International School. When she was 12 years old, her family returned to live in Australia. She attended Queenwood School for Girls in the Sydney suburb of Balmoral for her high school education.
Clemens began working as an actress in Australian television while in high school. She guest-starred in a 2006 episode of Blue Water High as Juliet, and in 2007, she starred in the children's series Pirate Islands: The Lost Treasure of Fiji as Alison. Clemens played Harper in the Showtime drama Love My Way that year, and was nominated for the Graham Kennedy Award for Most Outstanding New Talent at the 2008 Logie Awards for the role.
Clemens was seen in the 2008 MTV Networks Australia dramatic film, Dream Life, alongside Sigrid Thornton, Xavier Samuel, Linda Cropper and Andrew McFarlane. She had small roles in the television series All Saints, and the film X-Men Origins: Wolverine, in 2009. She became the face of Jan Logan's jewellery that year. Clemens relocated to Los Angeles, California in 2009.
Clemens starred in the 2010 film, Wasted on the Young, as Xandrie. Written and directed by Ben C. Lucas, the film told the story of a high school love triangle between Xandrie, Zack (Alex Russell), and Darren (Oliver Ackland), that leads to a party ending in gun violence. She guest-starred on the Fox crime drama, Lie To Me, opposite Tim Roth, and starred as a sociopathic prostitute in the film Generation Um..., alongside Keanu Reeves and Bojana Novakovic.
As of January 2010, Clemens was in negotiations to join the cast of Fury Road, a sequel to the Mad Max film series by George Miller. In 2011, she starred in the film Certainty, directed by Peter Askin, based on the play Searching for Certainty by Mike O'Malley. She also starred opposite Kevin Zegers and Keisha Castle-Hughes in Vampire, as Ladybird, a suicidal single mother. The film was the English-language feature debut of the noted Japanese director Shunji Iwai.
In 2012, Clemens starred in Camilla Dickinson, alongside Gregg Sulkin, Cary Elwes and Samantha Mathis. The drama was an adaptation of the 1951 novel of the same name by young adult fiction author Madeleine L'Engle. She portrayed teenager Heather Mason in Silent Hill: Revelation 3D. The horror film was based on the survival horror video game Silent Hill 3, and was a sequel to the film Silent Hill. That year, Clemens also played a lead role as the young suffragette Valentine Wannop in Parade's End, a television mini-series adaptation of the Ford Madox Ford tetralogy co-produced by HBO and BBC Two.
Clemens appeared in the 2012 horror film, No One Lives, starring Luke Evans. She also appeared in the 2013 film, The Great Gatsby, based on F. Scott Fitzgerald's novel of the same name. She played Catherine, the sister of Myrtle Wilson, played by Isla Fisher. Clemens starred as Tawney Talbot in the 2013 Sundance Channel television series, Rectify, created by Ray McKinnon.