While attending college, Switzer entered and completed the race in 1967 under entry number 261 with the Syracuse Harriers athletic club, five years before women were officially allowed to compete in it. Her finishing time of approximately 4 hours and 20 minutes was nearly an hour behind the first female finisher, Bobbi Gibb (who ran unregistered). She registered under the gender-neutral "K. V. Switzer", which she insists was not done in an attempt to mislead the officials. She claims to have long used "K. V. Switzer" to sign the articles she wrote for her college paper. She was issued a number through an "oversight" in the entry screening process, and was treated as an interloper once the error was discovered. Race official Jock Semple attempted to physically remove her from the race, and according to Switzer said, "Get the hell out of my race and give me those numbers." However, Switzer's boyfriend Tom Miller, who was running with her, shoved Semple aside and sent him flying. The photographs taken of the incident made world headlines.
After the race, Boston Athletic Association director Will Cloney was asked his opinion on Switzer's competition in the marathon. Cloney was quoted as saying, "Women can't run in the Marathon because the rules forbid it. Unless we have rules society will be in chaos. I don't make the rules but I try to carry them out. We have no space in the Marathon for any unauthorized person, even a man. If that girl were my daughter, I would spank her."
As a result of her run, the AAU barred women from all competition with male runners, on pain of losing the right to compete. Switzer, with other women runners, tried to convince the Boston Athletic Association to allow women to participate in the marathon. Finally, in 1972, women were welcome to run the Boston Marathon officially for the first time ever. Jock Semple, the man who had previously attempted to remove Switzer from the race, was instrumental in this formal admission of female runners.
Switzer was the women's winner of the 1974 New York City Marathon, with a time of 3:07:29 (59th overall). Her personal best time for the marathon distance is 2:51:37, at Boston in 1975.
Switzer was named Female Runner of the Decade (1967–77) by Runner’s World Magazine and received an Emmy for her work as a television commentator. She wrote Running and Walking for Women over 40 in 1997. She released her memoir, Marathon Woman, in April 2007 on the 40th anniversary of her first running the Boston Marathon. In April 2008, Marathon Woman won the Billie Award for journalism for its inspiring portrayal of women in sports. When visiting the Boston Marathon, Switzer is glad to see other female runners:
"When I go to the Boston Marathon now, I have wet shoulders—women fall into my arms crying."
They're weeping for joy because running has changed their lives. They feel they can do anything.
She was inducted into the National Women's Hall of Fame in 2011 for creating a social revolution by empowering women around the world through running. Since 1967, she has worked to improve running opportunities for women in different parts of the world.