Prior to entering politics, Yoneyama worked as a radiology researcher at the University of Tokyo Hospital and was also trained as a lawyer.

Whilst working towards his PhD, he had several attachments at the National Institute of Radiological Sciences and Harvard University, and did research at the Massachusetts General Hospital.

Despite the party doing well across the country in the election, he was defeated by the longtime incumbent and popular former Foreign Minister Makiko Tanaka.

Despite increasing his share of votes, Yoneyama was unable to overcome the Democratic Party of Japan landslide sweeping the country in the election.

Niigata was not spared by the strong anti-nuclear sentiments sweeping across the country, particularly due to the presence of the Kashiwazaki-Kariwa Nuclear Power Plant in the prefecture.

[5] His strong opposition pitted him against the pro-nuclear Abe administration, who threatened to cut Niigata's 1.2 billion yen (about $10.6 million) worth of grants if the reactors were not restarted.

He confessed to paying money to the women to simply win their favours, but acknowledged that it could create the impression of buying sex.

[8] His resignation triggered an early gubernatorial election in June 2018 and the nuclear debate was expected to heavily feature in the campaign.

[8] In the 2021 election Yoneyama ran as an independent candidate in the Niigata 5th district and won a seat in the House of Representatives.