Wieber started gymnastics at a young age. "I was probably two or three. My parents noticed that I had unusually bigger muscles than a typical little kid, so they put me in gymnastics. A little fun class where I could run around and play and stuff and just have fun. Then I actually stopped for a little bit to do dance class. When I was four I went back [to gymnastics] and that's kind of when I started getting really into it." She later enrolled with John Geddert, who handpicked her to his Silverstars program, and by age 10, she had qualified for Level 10 in the Junior Olympic program, and then qualified for international elite a year later in 2006.
Wieber rose to the Junior International Elite level in 2006, at age 11, where she placed ninth in the all-around at the 2006 National Championships. She was named to the US national team for the first time.