Fox was born in Toledo, Ohio. She recalls struggling with her birth-assigned gender as early as five or six years old. As a teenager, Fox believed she may have been a gay male, but learned about the term transgender at the age of 17. Fox continued living as a heterosexual male and married her then-girlfriend at the age of 19, when the latter became pregnant with their daughter. Fox then joined the US Navy to support her new family, serving as an operations specialist on the USS Enterprise.
After leaving the Navy, Fox enrolled at the University of Toledo, but dropped out citing ongoing psychological stress from her unresolved gender issues. After leaving college, Fox worked as a truck driver to earn money for sex reassignment surgery. She later moved to Chicago, Illinois with her daughter. In 2006, she traveled to Bangkok, Thailand to undergo gender reassignment surgery, breast augmentation, and hair transplant surgeries at the Bangkok National Hospital.
Fallon Fox came out publicly on March 5, 2013 in an interview with Outsports writer Cyd Zeigler and Sports Illustrated, after her two initial professional fights. Controversy swelled over confusion with the California State Athletic Commission (CSAC) and Florida’s athletic commission over the licensing process Fox chose to complete in Coral Gables. After publications shed light on the licensing procedure and Fox’s coming out many commentators brought up the issue of whether a person birth-assigned as a male should be able to fight in women’s divisions in MMA fighting.
UFC color commentator and stand-up comedian Joe Rogan came out in strong opposition to Fallon Fox receiving licensing, with Rogan stating “First of all, she's not really a she. She's a transgender, post-op person. The operation doesn't shave down your bone density. It doesn't change. You look at a man's hands and you look at a woman's hands and they're built different. They're just thicker, they're stronger, your wrists are thicker, your elbows are thicker, your joints are thicker. Just the mechanical function of punching, a man can do it much harder than a woman can, period.” Current state of research indicates that some trans women have less muscle strength and bone density than cis men, even though still higher than cis women, prior to medical treatment. The researchers concluded that this was likely due to a difference in lifestyle, not some innate biological difference in trans women.
Due to controversy and the licensing procedure CFA co-founder Jorge De La Noval, who promoted Fallon's fight on March 2 in Florida, postponed Fox’s April 20 fight. However, De La Noval later stated his organization will not “turn our backs on her… As long as she's licensed, she's always welcome in our promotion. We stand behind her and we give her all of our support." Fox claimed in her video interview with Cyd Zeigler to be within the rules of organizations like the International Olympic Committee (IOC) for postoperative transsexuals and wishes to continue fighting in MMA.
On 8 April 2013, Matt Mitrione, in an appearance on The MMA Hour, voiced several remarks in regard to Fox, including saying that Fox was "still a man", and calling Fox an "embarrassment" and a "lying, sick, sociopathic, disgusting freak". UFC “was appalled by the transphobic comments” of him, and, referring to itself as “a friend and ally of the LGBT community”, immediately suspended Mitrione and fined him an undisclosed amount. The next day Fox issued a response stating that Mitrione "personally attacked me as a fighter, as a woman, and as a human being".
The controversy over whether or not Fox possesses an advantage over other female fighters was a topic on the April 2014 edition of HBO's Real Sports with Bryant Gumbel.
In an interview with the New York Post, former UFC women's bantamweight champion Ronda Rousey stated she would be willing to fight Fox, saying "I can knock out anyone in the world", although she believes Fox has male bone density and structure, leading to an unfair advantage. In an interview with Out, Rousey said: "I feel like if you go through puberty as a 'man' it's not something you can reverse. … There's no undo button on that." UFC president Dana White claimed that "bone structure is different, hands are bigger, jaw is bigger, everything is bigger" and said "I don’t think someone who used to be a man and became a woman should be able to fight a woman.”
During Fox's fight against Tamikka Brents, Brents suffered a concussion, an orbital bone fracture, and seven staples to the head in the 1st round. After her loss, Brents took to social media to convey her thoughts on the experience of fighting Fox: “I've fought a lot of women and have never felt the strength that I felt in a fight as I did that night. I can’t answer whether it’s because she was born a man or not because I’m not a doctor. I can only say, I’ve never felt so overpowered ever in my life and I am an abnormally strong female in my own right,” she stated. “Her grip was different, I could usually move around in the clinch against other females but couldn’t move at all in Fox’s clinch…”
Eric Vilain, the director of the Institute For Society And Genetics at UCLA, worked with the Association of Boxing Commissions when they wrote their policy on transgender athletes. He stated in Time magazine that “Male to female transsexuals have significantly less muscle strength and bone density, and higher fat mass, than males” and was quoted as saying that, to be licensed, transgender female fighters must undergo complete “surgical anatomical changes […], including external genitalia and gonadectomy” (see sex reassignment surgery) and subsequently a minimum of two years of hormone replacement therapy, administered by a board certified specialist. In general concurrence with peer-reviewed scientific literature, he states this to be "the current understanding of the minimum amount of time necessary to obviate male hormone gender related advantages in sports competition". Vilain reviewed Fox's medical records and said she has “clearly fulfilled all conditions.” When asked if Fox could, nonetheless, be stronger than her competitors, Vilain replied that it was possible, but noted that "sports is made up of competitors who, by definition, have advantages for all kinds of genetics reasons". Fox herself responded to the controversy with an analogy in a guest editorial for a UFC and MMA news website: As an ethnic group, African American women statistically have bone mineral densities close to Caucasian males and bones more resistant to fracture, which, however, does not imply an unfair advantage of individual African Americans, either.
The documentary Game Face provides an inside look into Fallon's life during the beginning of her MMA controversy.
Fox was raised a devout Christian but has since become an atheist.
In 2014, Fox was inducted into the National Gay and Lesbian Sports Hall of Fame.