Her first acting role was Kate O'Neil in the made-for-TV movie Giant Mine. Then after a few minor roles, she had a role in the movie Custody of the Heart for which she was nominated for the 2001 Young Artist Award for Best Performance in a TV Movie (Comedy or Drama) for a Young Actress Age Ten or Under. In 2001, she lent her voice to the television series Committed. Over the next few years, she had roles in a few more made-for-TV movies, such as Harlan County War, One Kill, Range of Motion, and Jewel.
Arnold's career expanded in the mid-2000s. Beginning in 2005, she starred for three seasons as the title character of the popular Family channel series Naturally, Sadie. From 2006 until 2013, she played Holly J. Sinclair on Degrassi. She attended high school with co-star Munro Chambers.
In 2010, she won a Gemini Award for Best Performance in a Children's or Youth Program or Series, in a season 9 episode of Degrassi.