After graduating from university, Koizumi worked as a researcher at the Washington-based think tank Center for Strategic and International Studies, and became active politically as Young Leader of the Pacific Forum CSIS.

After the election of the Abe Government in 2012, Koizumi was appointed as a Vice-Minister for Reconstruction, focusing on the northeastern region of Japan that was devastated by the March 2011 tsunami and subsequent nuclear disaster.

Koizumi endorsed Taro Kono in the 2021 LDP leadership election, which resulted in Fumio Kishida becoming prime minister.

Koizumi ran as a candidate in the September 2024 Liberal Democratic Party presidential election to succeed Kishida and was the third most voted in the country, losing the first round to Sanae Takaichi and Shigeru Ishiba, with the latter leading the final runoff.

He spent one year as a part-time research fellow at the Center for Strategic and International Studies and as Young Leader of the Pacific Forum CSIS before returning to Japan in 2007.

[5] Koizumi became head of the LDP's young legislators caucus in October 2011, a post previously held by Prime Ministers Takeshita, Uno, Kaifu, Abe and Asō.

In the subsequent House of Councillors election in July 2013, he focused his campaigning efforts on disaster zones, outlying islands and areas in rapid population decline, giving speeches in support of their local LDP candidates.

Kenichi Tokoi, a nonfiction author who wrote a book about Koizumi, said that his goal was to shake as many individual hands as possible and to leave the impression that he was kind enough to visit them, something which he could not achieve by campaigning in big cities.

Ishiba, then secretary general of the LDP, stated that Koizumi "made a very strong case" with local disaster victims "about what he wanted to do and why.

[25] After Kōno also declined to run Koizumi and Kono both endorsed Chief Cabinet Secretary Yoshihide Suga, a fellow Kanagawa politician.

[28] Poor approval ratings and criticism of the government's handling of the COVID-19 pandemic led to Suga announcing his resignation in September 2021.

He was chosen to chair the standing committee on national security of the House of Representatives in January 2024 after the previous chairman resigned in connection to the 2023–2024 Japanese slush fund scandal.

[36] On 14 August, Prime Minister Fumio Kishida announced he would not seek a second term as LDP President, making the race an open field.

[41][42][43] In a press conference he pledged to introduce legislation that would legalize separate surnames for married couples and proposed holding a national referendum to determine whether or not Article 9 of the Constitution should be amended.

As LDP president, Ishiba appointed Koizumi chairman of the Election Strategy Committee, a senior party office.

[51] In the October 2024 election Koizumi traveled across the country to rally support for LDP candidates, but the results were unfavourable, with the ruling coalition losing its majority.

[54] When asked in February 2025 about the possibility of running for party leadership again, Koizumi didn't rule it out, saying "I will do my best to become a politician that people want to support.

"[55] Like his father, Koizumi visits Yasukuni Shrine on 15 August, the anniversary of Japan's surrender in World War II.

[59] In a May 2013 interview with the Sankei Shimbun, he refused to comment on Osaka mayor Toru Hashimoto's controversial remarks on comfort women, characterizing the issue as one that should be discussed among experts and historians rather than politicians.

He described the perceived nationalist shift in Japanese politics as "Chinese propaganda" and stated that the government needed to wage a better public relations campaign against it while focusing on the successful implementation of Abenomics.

[69] Following the December 2012 election, the National Diet Building gift shop began selling "Shinji-Rolls" (進次ろうる), souvenir green tea-flavoured roll cakes branded with Koizumi's likeness.

[72] In January 2020, Koizumi received international news attention when he announced his plans to take two weeks of paternity leave when his first child was born.