[4] Shimomura held a seat in the Diet for nearly three decades, winning nine consecutive terms until losing in the 2024 Japanese general election.

[8] As minister, Shimomura expanded the ability of local schools to provide Saturday classes[9] and pushed for the globalization of Japanese universities by increasing English skills and hiring more foreign faculty.

[13][14] Following the resignation of Tokyo governor Naoki Inose on 19 December 2013, Shimomura was widely rumored to be a potential candidate for the gubernatorial election expected to be held in February 2014, along with Yuriko Koike, Hideo Higashikokubaru, Seiko Hashimoto and Yoichi Masuzoe.

Prior to the 2012 general election, Shimomura told the magazine Apple Town that "we intend to construct a genuinely conservative administration... [such that] has only existed for about three years since the war", describing tensions with China and South Korea as a "national crisis" and stating that "the 67 years since the end of World War II have been a history of Japan’s destruction.

"[18] Following his appointment as education minister in 2012, he refused to comment on the issue but stated that "the government has decided to study Japan's interpretation of history, including the Kono Statement [acknowledging the army's involvement], and I would like to express my views during that process, if necessary.

Shimomura commented to the New York Times that his son would not have been able to enroll in a Japanese university, and said "the British system is more open to a broader range of people and talents...

"[10] In April 2018, Shimomura called it "almost a crime" for the TV reporter who complained of sexually harassment (explicit language) to have secretly taped the former Ministry of Finance top official Junichi Fukuda [ja] who was her alleged victimizer and "selling" the story to a tabloid weekly.

Although he retracted his choice of words the next day, he still insisted he had a point to make in that the reporter violated the journalistic principle of speaking off-the-record, and he spoke out because he suspected her of staging this with the intention of selling the story to the tabloids from the beginning.