She became Japan's first ever female defense minister in 2007, and a year later became the first woman to run for leader of the Liberal Democratic Party. And, earlier this month, she was elected Tokyo's first ever female governor by a landslide.
But breaking through hasn't always been easy. In Japan, she says the glass ceiling is more like steel -- and now she wants to help women beneath it.
"The potential of Japan is so enormous," she told CNN in an interview in her Tokyo office.
"But half of the population -- women -- is not well used. I want to do my best to make women in Japan and Tokyo (make better) use of their ability."
Aging, male-dominated society
Japanese society is heavily male-dominated in both politics and business. The World Economic Forum ranks the country 101st out of 145 in terms of gender equality, and only 9.5% of Japan's House of Representatives are women, according to the Inter-Parliamentary Union.
One of the main problems, Koike says, is childcare: too many women are forced to make a choice between having a career and building a family.
"It's not fair," she says. "In the US and other countries, women have more freedom to choose both."
She vowed in her gubernatorial campaign to improve the city's childcare system and get more women into the workforce.
Doing so will also help tackle another major problem facing Japan: its rapidly aging population. Today, more than 25% of the country's population is over 65, according to the UN. By 2060 that figure is expected to have risen to 40%.
"Demography is one of the biggest social issues (facing Japan)," Koike says. "In such a gigantic city like Tokyo, we have to find out a good solution for that."