Having aspirations to join the Navy from an early age, in 1900 he graduated from high school and enrolled at the Imperial Japanese Naval Academy on 17 December.

As Hasegawa had graduated on the eve of the Russo-Japanese War, his class did not make the usual long-distance navigational training voyages, which only resumed after the conflict.

He graduated on 27 May 1914, ranking second in his class of 16, and briefly commanded the destroyer Mikazuki before being assigned as the aide to the admiral of the Second Fleet.

Hasegawa was promoted to naval attaché in Washington on 20 March 1919, and returned to Japan the following year, resuming service in the Personnel Department of the Navy Ministry.

This cemented his friendship with a fellow captain and future vice-admiral, Terashima Ken (1882 - 1972); they remained friends until Hasegawa's death.

An anecdote is given that at the new Governor-General's official welcoming ceremony; he picked up a maid in a burst of exuberance and sat her on his lap, which astonished many of those present.

During Hasegawa's tenure as Governor-General, a preparatory course for Taihoku Imperial University was set up and compulsory education at the elementary level strengthened.

He moderated the radical Kominka (Japanization) movement in Taiwan promoted by his predecessor, Seizō Kobayashi, who wanted to replace Taiwanese folk religion by Japanese Shinto.

Along with many other leading politicians and military commanders, Hasegawa was arrested in late 1946 as a suspected Class A war criminal by the American occupation authorities.

However, Hasegawa made a formal apology to the American and British officers; impressing the court with his integrity, he was acquitted.

Following his release from Sugamo Prison on 14 January 1947, in 1951 he sat on an advisory committee composed of former Imperial Japanese Navy officers to oversee the formation of the Japan Maritime Self-Defence Force.