Marshall made her acting debut in 2000 in a television series called Super Rupert. The following year, she starred in an episode of Twice in a Lifetime. She next performed in the television series Doc (2002) and Dark Oracle (2004), and in the television film The Christmas Shoes (2002).

In 2003, she was nominated for a Los Angeles-based Young Artist Award for Best Performance in a TV Movie, Miniseries or Special for Leading Young Actress for The Elizabeth Smart Story, a movie-of-the-week based on the true story of the 2002 kidnapping of Utah teenager Elizabeth Smart.

In 2007, Marshall was cast as Amy Fleming in the forthcoming CBC series Heartland. The series is based on the 25-novel series written by Lauren Brooke. The series premiered on Canadian television in October 2007 and its twelfth season aired in 2019. The series, filmed in High River Alberta, is about a girl named Amy Fleming living in the fictional town of Hudson Alberta. Her family owns a horse ranch called Heartland, where they have a long tradition of healing horses. For her performance in the series, Marshall won the inaugural Canada’s Screen Star Award at the 1st annual Canadian Screen Awards. In 2015, Heartland surpassed Street Legal as the longest-running one-hour scripted drama in the history of Canadian television.

As of 2016, she lives on a farm ranch outside Calgary, Alberta with her husband Shawn Turner and their animals. Marshall married Shawn, a photographer, on July 27, 2013 after they got engaged in Spring 2012.