This was Switzer's 40th marathon and her ninth time running the Boston race. The 70-year-old walked through water stations, stopped for pictures and interviews and still finished under qualifying time: 4:44:31 and an average mile of 10:51.
The 1967 encounter was captured in an iconic photo that turned Switzer into a hero and launched her career as an advocate for women in sports.
Switzer has said she did not intend to break barriers by entering the race. After all, another woman, Roberta Bingay Gibb, had completed the Boston Marathon in 1966 without a bib. But the photo exposed the ugly nature of sexism in sports, thrusting Switzer into the spotlight.
"What happened on the streets of Boston 50 years ago completely changed my life and changed other people's lives," she said in a phone interview after the race. "The race today was a celebration of the past 50 years; the next 50 are going to be even better."
'No dame ever ran the Boston Marathon'
Plenty has changed in running since 1967, thanks in part to Switzer's efforts. To start, women are welcome in the Boston Marathon and other major races.
Unlike Gibb, Switzer managed to score a bib in 1967 by signing up with her initials, K.V. Switzer. As she tells it, there were no official written rules saying only men could enter the race. Nor was there a spot on the entry form to select gender.
But in those days, women rarely participated in professional or competitive sports. Even her coach at Syracuse -- where Switzer trained with the men's cross-country team -- told her the distance was too long for "fragile women."
"No dame ever ran the Boston Marathon!" coach Arnie Briggs told her, according to her memoir, "Marathon Woman
." But if she could run the distance in practice he promised to take her to Boston.
When Switzer completed the 26-mile trial, Briggs insisted she sign up officially. She said she used her initials because her first name was misspelled on her birth certificate, Kathrine, and she was tired of repeating the error. Plus, she said she wanted to be a writer, and using her initials, like J.D. Salinger and e.e. cummings, seemed like a "cool, writerly" thing to do.
Her bib number would come to represent fearlessness in the face of adversity for female runners ever since. The Boston Marathon will retire number 261 in Switzer's honor.
'Give me those numbers!'
Switzer said she did not try to hide the fact she was a woman. She wore lipstick, earrings and burgundy shorts, but ended up wearing baggy sweats over her "feminine" running gear because of the wintry weather.
It was snowing by the time she and her teammates reached the starting line in Hopkinton. One of them told her to wipe off her lipstick so organizers would not notice her. She refused and began the race.
A few miles in she saw a man with a felt hat and overcoat in the middle of the road shaking his finger at her as she passed. Then, she heard the sound of leather shoes, a distinctly different noise from the patter of rubber soles, and knew something was wrong.
"Instinctively I jerked my head around quickly and looked square into the most vicious face I'd ever seen. A big man, a huge man, with bared teeth was set to pounce, and before I could react he grabbed my shoulder and flung me back, screaming, 'Get the hell out of my race and give me those numbers!'" she wrote in her memoir.
The man was race director Jock Semple. Press photographers captured Semple's contorted face as he grabbed at Switzer's numbers while her boyfriend pulled Semple off her.
She ran from the scene bewildered. She ambled on for a few miles before her anger transformed into energy and she took off for finish line. Dropping out was not an option, she said on her website
"...I knew if I did that no one would believe women could run distances and deserved to be in the Boston Marathon; they would just think that I was a clown, and that women were barging into events where they had no ability. I was serious about my running and I could not let fear stop me," she said.
She finished the race in four hours and 20 minutes, but would later be disqualified and expelled from the Amateur Athletic Union.
Support soon eclipsed the fallout and she became a celebrity.
'I could not let fear stop me'
Switzer used her influence to campaign to get women into the Boston Marathon by 1972. She went on to run 39 marathons, and achieving her personal best in 1975, 2:51:33, when she finished second in Boston. She won the New York City Marathon in 1974 in a moment that was captured by legendary photographer Ruth Orkin
She created the Avon International Running Circuit of women's-only races in 27 countries, paving the way for the first women's Olympic marathon in 1984. She became an author and TV commentator for the Olympics, World and National championships before returning to marathons at 64.
Along the way, 261 became a rallying cry among female runners. Switzer formed 261 Fearless
, a nonprofit running club for women that has groups across the country. Some of its members will join Switzer for her victory lap on Monday, wearing the 261 bib to raise money for charity.
Photographers swarmed Switzer on Monday as she crossed the finish line and a race official placed a medal around her neck.
"Running is a social revolution now. Women are not just doing it to get into races or to lose a couple of pounds, they're doing it for fun, for their self esteem. It's transformative," she said.
"We've come a light year but we still have a long way to go."