Nils Allen Stewart Jr. was born in Beverly Hills, California. His father, Nils Allen Stewart, is a professional stuntman. His younger sister, Fivel, is also an actress and recently appeared in the second season of the Netflix original dramedy Atypical. His mother has Japanese, Chinese, and Korean ancestry & his father has Russian, Scottish, and Blackfoot ancestry. Stewart was a former member of T-Squad, a Disney hip-hop/pop group, and performed with his sisters, Fivel and Maegan, as part of "TSC" (The Stewart Clan).