[2][9] Thomas began to question her gender identity towards the end of high school, and came out as transgender to her family after her freshman year at college, during the summer of 2018.

[12] Thomas began transitioning using hormone replacement therapy in May 2019, and came out as a trans woman during her junior year to her coaches, friends, and the women's and men's swim teams at the University of Pennsylvania.

[31] In March 2022, Sports Illustrated reported that Thomas applied for law school and planned to swim at the 2024 Summer Olympics trials.

[33] In June 2022, the International Swimming Federation (FINA), an organization that administers international aquatic sports competitions, voted to bar all transgender athletes from competing in professional women's swimming, with the exception of athletes who "can establish to FINA's comfortable satisfaction that they have not experienced any part of male puberty beyond Tanner Stage 2 (of puberty) or before age 12, whichever is later".

[2][4][39][40] In early December, anonymous parents of University of Pennsylvania swim team members wrote to the NCAA, seeking for Thomas to be declared ineligible to compete.

[2] In December 2021, USA Swimming official Cynthia Millen resigned after 30 years in protest against Thomas's eligibility to compete and then she appeared to express her views on the Fox News show The Ingraham Angle.

[42][43] In February 2022, in response to a proposed NCAA transgender athlete policy that could prevent Thomas from competing in the NCAA championships, sixteen anonymous members of the University of Pennsylvania women's swimming team sent a letter to the university and Ivy League officials asking them not to take legal action against the proposal.

[4] Brooke Forde, an Olympic silver medalist, said of Thomas that "I believe that treating people with respect and dignity is more important than any trophy or record will ever be, which is why I will not have a problem racing against Lia at NCAAs this year".

[46][47] Another swimmer, Olympic silver medalist Erica Sullivan, spoke in support of Thomas in an opinion piece for Newsweek: "like anyone else in this sport, Lia has trained diligently to get to where she is and has followed all of the rules and guidelines put before her ... she doesn't win every time.

[4][49] Three-time Olympic gold medalist Nancy Hogshead-Makar opposed Thomas's participation, arguing that she had not "demonstrated that [she] lost her sex-linked, male-puberty advantage prior to competition in the women's category".

[4] In March, roughly 50 protesters and counter-protesters gathered outside the Georgia Tech Aquatic Center when Thomas swam in the NCAA Division I national championship.

", while social media users responded by pointing out Jenner has previously expressed support for transgender athletes and has competed in women's golf.

"[59] As of June 2022, Alabama, Arizona, Arkansas, Florida, Georgia, Idaho, Indiana, Iowa, Kentucky, Louisiana, Mississippi, Montana, Oklahoma, South Carolina, South Dakota, Tennessee, Texas, Utah, and West Virginia have laws prohibiting public schools from allowing the participation of transgender girls in school sports for girls.

[62] The National Women's Law Center, a non-profit organization, defended Thomas, saying that she "deserves all the celebration for her success this season, but instead is being met with nationwide misogyny and transphobia".

[64][65] In June 2024, the Court of Arbitration for Sport ruled that Thomas did not have standing to challenge the policy, meaning she would remain ineligible to compete.