A former Japanese defense minister, the 64-year-old received over a million votes more than her nearest rival, Hiroya Masuda, who was backed by Prime Minister Shinzo Abe's ruling Liberal Democratic Party.
Koike, who previously broke glass ceilings as Japan's first female defense chief, is one of only three women currently serving as governor of one of the country's 47 prefectures.
In her campaign, Koike referenced Joan of Arc and anime character Sally the Witch, and vowed to overcome childcare shortages and push female-friendly policies
"so that both women and men can shine in Tokyo."
Japanese society is still 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.
The status of women at the very top of Japanese society has also come into question in recent years. Current laws do not allow for a woman to accede to the Chrysanthemum Throne. Princess Aiko would be second-in-line to succeed Emperor Akihito, after her father, Crown Prince Naruhito. Unless the law is changed however, the title will pass on Naruhito's death to Aiko's uncle, Prince Fumihito.
A graduate of Cairo University, Koike is fluent in both English and Arabic, and worked as a translator and newscaster before entering politics.
Outspoken and opinionated, she is a frequent commentator on Japanese and global affairs. In recent months she has praised
Taiwan's recently elected president Tsai Ying-wen and warned of Donald Trump's "potential to do lasting damage" to the U.S.-Japan relationship.
Koike was endorsed for Tokyo governor by the Japan Society for History Textbook Reform
, which promotes a revisionist, nationalistic view of Japanese history, particularly downplaying or whitewashing war crimes and the use of sex slaves by Japanese forces in WWII. In the past, she's written
about the need for Japan to "take responsibility for the future, not obsess about the past."
She is also a frequent critic of North Korea, previously accusing
Koreans in Japan of sending money to support Kim Jong Un's regime.
Environment Minister from 2003 to 2005, Koike stressed her green credentials during her gubernational campaign, encouraging supporters to wear green and donning a green headband herself for photo-ops.
In 2005 she pioneered a widely-adopted program
to encourage male office workers to ditch their suit jackets in summer, allowing office air conditioners to be set at higher temperatures.
Koike has also promoted the introduction of a carbon tax and the use of forshiki -- Japanese wrapping cloth -- shopping bags in place of plastic.
Her Twitter handle is @ecoyuri
As governor of Tokyo, Koike will oversee preparations for the 2020 summer Olympic Games.
Despite still being four years away, the games have already attracted controversy, with construction of the main stadium delayed after a high-profile plan by the late architect
Zaha Hadid was scrapped at the last minute
due to rising costs.
The games' official logo also had to be dropped after it was alleged that the design was plagiarized
The 2020 games are "a defining moment -- an occasion for Tokyo's citizens to consider how we want to be seen, and how we see ourselves," Koike wrote