Millicent Simmonds

Simmonds grew up in Bountiful, Utah in the United States. She has four siblings; two older and two younger than her. When Simmonds was two months old, an accidental medication overdose caused permanent hearing loss for her. Her mother learned American Sign Language and taught the family so they could communicate with her. Simmonds said without her family using ASL, "I wouldn't have a relationship with my own family, I wouldn't have communication." Simmonds also has a cochlear implant.

Simmonds's mother also encouraged her to read books extensively. When Simmonds was three years old, she started attending the Jean Massieu School of the Deaf, and around third grade, she started its drama club. Her first play was in A Midsummer Night's Dream as Puck. After completing sixth grade, she mainstreamed at the Mueller Park Junior High School in the fall of 2015. She has performed at the Utah Shakespeare Festival in Cedar City, Utah, and her primary film experience before Wonderstruck was a deaf student's short, "Color the World".

In the third quarter of 2020, Simmonds's step-father got a job promotion, and Simmonds and her family moved from Bountiful, Utah to a neighborhood north of Pittsburgh, Pennsylvania.