Mosteller was born in New York City, NY. The youngest of five children to Rosa Mosteller (née Cedarbrook). He has a baby sister named Rose Josie Mosteller (born July 10, 1996) Rose Mosteller is also an actress. He has another younger sister named Maisey Hattie Mosteller (born July 14, 1996) He has an older sister named Josie McKenna Mosteller (born July 1, 1996) He spent most of his early life and childhood in Scotland and then moved to Colorado. He started acting at the age of three. In 2011, he started filming for Judy Moody and the Not Bummer Summer. He played the role of Stink.