Brenda Song was born in Carmichael, California, a suburb of Sacramento, to a Hmong father and a Thai mother who had been adopted by a Hmong family. Her paternal grandparents were from the Xiong clan, but changed their last name to Song when the family immigrated to the United States. Her parents were born in Asia and met as adults in Sacramento. Her father works as a schoolteacher and her mother is a homemaker. She has two younger brothers, Timmy and Nathan.
When she was six years old, Song moved with her mother to Los Angeles to support her acting career; the rest of the family followed two years later. As a young girl, Song wanted to do ballet, while her younger brother wanted to take taekwondo. She said, "My mom only wanted to take us to one place," so they settled on taekwondo. Although Song cried all the way through her first class, she now holds a black belt in taekwondo. Song was named an All-American Scholar in the ninth grade. She was homeschooled and earned a high school diploma at age 16, then took courses at a community college and online from the University of California, Berkeley, with a major in psychology.