He was educated in military preparatory schools in Kobe and Osaka and graduated from the 20th class of the Imperial Japanese Army Academy in 1908, whereupon he was assigned to the IJA 28th Infantry Regiment.

In 1932, Sakai was promoted to colonel and was assigned to the 5th Section military intelligence of the 2nd Bureau of the Imperial Japanese Army General Staff from 1932 to 1934.

On December 8, 1941, a few hours after the attack on Pearl Harbor, Japanese forces commanded by Sakai, and his Chief of Staff Tadamichi Kuribayashi, invaded Hong Kong.

However, the subsequent Battle of Hong Kong did not proceed as quickly or as smoothly as Sakai had planned, and he was forced to request an extension to his deadline.

[3] He was convicted at the Nanjing War Crimes Tribunal on August 27, 1946 of command responsibility for the extrajudicial murder of Chinese civilians and was executed by firing squad on September 30.