Later in life, Semple reversed his position on women competing in the marathon.

Photographs of Semple attempting to rip Switzer's number off were widespread in the media. Switzer wrote in her memoir '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!'' Switzer's boyfriend managed to shove Semple aside. When another, Kathrine Switzer, entered the race officially, through an 'oversight' in the entry screening process, Semple tried to stop her as she ran.

In the 1967 Boston Marathon, one woman, Bobbi Gibb, ran and finished unofficially, as she had the previous year, because women were not allowed to participate.

Jock's method of attack is apt to vary.' In 1957, Semple had narrowly escaped arrest for assault after attempting to tackle a runner in swim fins and a snorkeling mask. In a 1968 interview with Sports Illustrated, he called them 'These screwballs! These weirdies! These MIT boys! These Tufts characters! These Harvard guys!' According to fellow race official Will Cloney: 'He hurls not only his body at them, but also a rather choice array of epithets. He had been in the long-time habit of physically attacking those he perceived to be 'non serious' runners competing in the race, whether officially entered or running the course unofficially. Semple became known to a lay audience while working as a Boston Marathon race co-director. He was a masseur and physical therapist for the Boston Bruins and the Boston Celtics, and a trainer for Olympic athletes. He moved to Boston after running in his first Boston Marathon and began to work in sport-related fields. Semple was born in Glasgow, Scotland and emigrated to the United States in 1921 to work as a cabinetmaker in Philadelphia.

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