Lowndes got her acting debut in the 2005 TV movie, Saving Milly as Andrea Kondracke. This was followed by a guest role in an episode of Masters of Horror, she was later cast in a recurring role on the sitcom Alice, I Think. She has since made guest appearances in Kyle XY. Her next film role was a supporting role in the Lifetime movie To Have and to Hold released in 2006, this was followed in 2007 by a pilot called Pretty/Handsome which did not get picked up.
In 2008 Lowndes appeared in Autopsy and The Haunting of Molly Hartley along with a recurring role in the drama series Greek as Mandi.
In 2008 Lowndes landed a recurring role in the series 90210 a spin-off of the 90s TV teen drama Beverly Hills, 90210 as Adrianna Tate-Duncan. In November 2008 her role was upgraded to series regular.
Lowndes starred in the Canadian horror film Altitude in 2010.
