Muniz was born on December 5, 1985, in Wood-Ridge, New Jersey. His father, Francisco III, was a restaurateur, and his mother, Denise, was a nurse. His father is Puerto Rican and his mother is of Italian and Irish descent. He has an elder sister, Cristina. When Muniz was four years old, his family moved to Knightdale, North Carolina, where he grew up. He was first discovered at the age of eight, at a talent show in Raleigh, North Carolina, in a local production of A Christmas Carol, cast as Tiny Tim. Shortly after, his parents divorced, and he subsequently moved to Burbank, California, with his mother. Beginning in the sixth grade, Muniz was home-schooled by his mother. He appeared in commercials and made his film debut in the made-for-television movie To Dance With Olivia (1997), starring Lou Gossett Jr. In that same year, he appeared in the CBS Hallmark Hall of Fame presentation of What the Deaf Man Heard. A small role in the film Lost & Found (1999) led to his breakout role as the title character on Malcolm in the Middle.