During his tenure he oversaw the formation of the Liberal Democratic Party (LDP) and restored official relations with the Soviet Union.

After graduating from Tokyo Imperial University, he practiced law before entering political life, and was first elected to the Diet in 1915 as a member of the Rikken Seiyūkai.

He served as chief cabinet secretary under Giichi Tanaka from 1927 to 1929, and minister of education under Tsuyoshi Inukai and Makoto Saitō from 1931 to 1934.

During his tenure, Hatoyama attempted to push through an electoral reform to ensure a two-party system in order to revise the constitution's pacifist Article 9, but failed in both efforts.

In 1956, he restored diplomatic relations with the Soviet Union, ending the formal state of war which had existed since 1945, and secured Japan's entry into the United Nations before he resigned.

Kisaburo Suzuki, the husband of Hatoyama's elder sister Kazuko, served as Minister of Justice in the new cabinet.

In June Kiyoura had to resign in favour of Takaaki Kato, who had formed a coalition of his own Kenseikai, the Seiyukai and the Kakushin Club.

The Seiyūhontō moved towards coalition with the Kenseikai, but Hatoyama opposed this and left the party with about twenty Diet members in December.

Inukai was assassinated in the May 15 incident and Suzuki was elected to succeed him as Seiyukai president, but he didn't become prime minister, as the genrō Prince Saionji preferred to nominate Admiral Makoto Saito.

Hatoyama continued in his post and became involved in a controversy in March 1933 when he had a professor at Kyoto Imperial University dismissed for leftist views.

In March 1934 he was forced to resign due to alleged corruption in the Teijin Incident, which eventually led to the downfall of the Saito cabinet.

[6][9] When Suzuki was once again passed over as prime minister, this time in favour of Admiral Keisuke Okada, the Seiyukai moved into the opposition, even expelling members who accepted positions in the new cabinet.

Shared opposition to the Tojo cabinet brought him together with his old rival Bukichi Miki, who also ran and won as a non-endorsed candidate.

When fellow Diet member Bin Akao was expelled from the Imperial Rule Assistance Political Association for publicly rebuking Tojo in June 1943, Hatoyama and others decided to leave in solidarity.

This practically made further political activities impossible, so Hatoyama decided to retire to his summer house in Karuizawa.

In Karuizawa he devoted himself to farming and study, but he also took part in plans surrounding Shigeru Yoshida and Fumimaro Konoe for an early peace with Britain and the United States.

[12] Shojiro Ishibashi was a significant sponsor for the new party and his house would serve as a de facto office in the early days.

Hatoyama was expected to become prime minister, but while in the middle of putting together his cabinet, he was purged from public office by the occupation authority on 4 May.

[15] As part of the so-called "Reverse Course," in American policy towards Japan, brought about by the increasing confrontation between the United States and the Communist bloc, many people were released from the purge, including Miki and Kono.

[20] In November 1954, Hatoyama formed the Japan Democratic Party by merging the non-mainstream factions of the Liberal Party with the Kaishintō under Mamoru Shigemitsu, with Hatoyama as president, Shigemitsu as vice president, Nobusuke Kishi as secretary general and Miki as general council chairman.

[22] Hatoyama favored pardons for some of the Class A war criminals who had been sentenced to life imprisonment by the Tokyo Trial.

[25] That same year, Hatoyama attempted to implement his infamous "Hatomander" (ハトマンダー, hatomandā, a portmanteau of Hatoyama and Gerrymander), an attempt to replace Japan's SNTV multi-member constituencies with American-style first-past-the-post single-member districts, which would have made it easier for the LDP to secure the two-thirds of seats in the Lower House of the National Diet needed to revise the Constitution.

[28] Iichirō Hatoyama, Ichirō's only son, made a career for himself as a civil servant in the Budget Bureau of the Finance Ministry.

During the purge against Ichirō (1946–1951), he received an English book The Totalitarian State against Man originally written in German by the half-Japanese Austrian Count Richard von Coudenhove-Kalergi from a professor of Waseda University Kesazō Ichimura (1898–1950) who wanted Ichirō to translate the English book into Japanese.

[30] The English book struck a sympathetic chord in Ichirō, and he began to advocate fraternity, also known as yūai (友愛) in Japanese.