Simmons was born in Vancouver, British Columbia, the middle child of three (one younger brother and an older sister). Her first movie role was as Pennywise's first victim in the horror movie It (1990) in which she played Laurie Ann Winterbarger. Her most recognizable roles are in movies such as Final Destination 3 (2006) as Ashley Freund, and also in other movies like John Tucker Must Die (2006) and Good Luck Chuck (2007). She has also guest starred in episodes of Smallville, Mutant X, and Supernatural. Chelan portrayed Hilary on the ABC Family show, Kyle XY for 25 episodes. She portrayed a Lotus Eater in the 2010 film Percy Jackson & the Olympians: The Lightning Thief.