Mitchell was born in Mississauga, Ontario, the daughter of Precious Garcia and Mark Mitchell. Her parents work in finance. Her father is of Scottish and Irish descent and her mother, who is Filipino, is from the province of Pampanga, and left the Philippines at age 19. She has one younger brother named Sean. She is a first cousin-once-removed of Lea Salonga, a singer and Tony Award-winning theater actress well known for originating the lead role of Kim in the musical Miss Saigon. Even as a young child, Mitchell was interested in performing arts, declaring to her parents that she would pursue such a career. Mitchell began dance lessons at the age of five, and she and several of her classmates toured the city to compete with a variety of other dance schools. At the age of 10, Mitchell's family moved to West Vancouver, British Columbia. Within a year of their arrival, an international modeling agency held an open casting call for teen and pre-teen girls who were vying for representation. Mitchell was one of the girls selected. She attended Rockridge Secondary School and then later transferred to West Vancouver Secondary School where she graduated. She got the nickname Shay because she is a big fan of Jennifer Lopez aka J. Lo, so her friends called her "Shay Lo".