Born in Selma, Alabama, Mia was the fourth of six children of Bill and Stephanie Hamm. She wore corrective shoes as a toddler after being born with a club foot. Hamm spent her childhood on various United States Air Force bases around the world with her family. While living in Florence, Italy, Hamm first played soccer, which was hugely popular there; her entire family quickly became involved in the sport.

In 2001, Hamm was a founding player in the first professional women's soccer league in the United States, the Women's United Soccer Association (WUSA), and played for the Washington Freedom from 2001-2003. Throughout the league's history, Hamm was hailed as the star of the league and used heavily in marketing and promotion. In a poll of 1,000 advertising executives conducted in 2001, she was voted "the most appealing female athlete", garnering almost twice as many votes as the runner-up Anna Kournikova.  During the league's inaugural match between the Freedom and Bay Area CyberRays at RFK Stadium in Washington, D.C., Hamm was fouled in the penalty area resulting in a penalty kick that her teammate Pretinha converted to mark the first goal scored in the league. The Freedom won 1-0. In addition to the 34,148 fans in attendance being greater than any MLS game that weekend, the Turner Network Television (TNT) broadcast reached 393,087 households: more than two MLS games broadcast on ESPN and ESPN2. Playing as a midfielder and forward, Hamm played in 19 of the Freedom's 21 matches during the 2001 season. She led the team in goals (6) and assists (4). The Freedom finished in seventh place during the regular season with a 6-12-3 record.  Hamm suffered a knee injury in November 2001 that kept her off the pitch for several months of early 2002. Despite playing only half the 2002 season with the Freedom, she finished the season with eight goals. The team finished in third place during the 2002 season with a 11-5-5 record and advanced to the playoffs. After winning the semi-final against the Philadelphia Charge 1-0, the team was defeated 3-2 by the Carolina Courage in the 2002 WUSA Founders Cup. Hamm scored the Freedom's second goal in the 64th minute.  During the 2003 season, Hamm started in 16 of the 19 games in which she played. Her 11 goals ranked second on the team behind Abby Wambach's 13 while her 11 assists ranked first. The Freedom finished in fourth place during the regular season with a 9-8-4 record and secured a berth in the playoffs. Hamm finished her club career as a WUSA champion when the Freedom defeated the Atlanta Beat 2-1 in overtime to win the Founders Cup on August 24, 2003.

Did the team do well without her?
The team finished in third place during the 2002 season with a 11-5-5 record and advanced to the playoffs.