Katelyn Mager was born in Vancouver, Canada.

She is best known for role as Young Thalia, in the Feature Film Percy Jackson:Sea of Monsters (2013), which won her a 2014 Joey Award as well as a nomination for the Best Scene Stealer at the 2014 Kids Pick Flick Awards. Katelyn is also well known and loved by the fans (Hearties) for her role as Rachel Stonelake in the Hallmark Television Series When Calls the Heart.

Katelyn's film career began in 2009 when Katelyn appeared in the Feature Film Damage with Steve Austin. Following this, she played the role of Dolph Lundgren's daughter Taylor in the Killing Machine (2010). Also in 2010, Katelyn appeared in the television series's Supernatural and Life Unexpected. In 2011, Katelyn played the challenging role of Lissa an Asperger's patient in television movie Normal(2011),opposite Carrie-Anne Moss. In 2012, Katelyn co-stared in the film Floodplain with Cameron Bright (Twilight) that premiered at the Cannes Film Festival, and also won Katelyn her second Joey Award. In 2015, Katelyn played the lead role of Charlotte in the Dark Fantasy Feature Film Charlotte's Song., which was premiered at the Vanocuver International Film Festival  and garnered her another Joey Award for lead female in a Feature Film. As well, Katelyn played the lead role of Angel in the film Discovered, which won her a Young Artist Awards for Lead female.