With the death of his sister-in-law Kikuko, Princess Takamatsu, on 17 December 2004, Prince Mikasa became the oldest living member of the Imperial House of Japan.

Upon attaining the age of majority in December 1935, Emperor Shōwa granted him the title Mikasa-no-miya (Prince Mikasa) and the authorization to form a new branch of the imperial family.

[4] In a 1994 interview, he criticized the Imperial Army's invasion of and atrocities in China, and recalled having been "strongly shocked" when an officer informed him that the best way to train new recruits was to use living Chinese POWs for bayonet practice.

[6] In one of Prince Mikasa's memoirs, he wrote that he toured Unit 731's headquarters in China and was shown films showing Chinese prisoners "made to march on the plains of Manchuria for poison gas experiments on humans.

[8] After the war, it was reported that while an officer, Prince Mikasa had taken a strict stance against lax discipline and the cruel actions of Japanese soldiers serving in China.

[11] Out of a desperate desire to end the war, he wrote and delivered a speech that condemned Japanese troop aggression against the Chinese, elaborating that repeated rape, plunder and killing of civilians created strong anti-Japanese feelings in China, and that the puppet government in Manchukuo was an attempt to cover up Japan's policy of aggression in China.

[12] He also disclosed that the Japanese served fruit contaminated with cholera germs to a team from the League of Nations that came to investigate Japan's invasion of China.

His role was intended to bolster the legitimacy of the Wang Jingwei regime and to coordinate with Japanese Army staff towards a peace initiative, but his efforts were totally undermined by the Operation Ichi-Go campaign launched by the Imperial General Headquarters.

[17] Prince Mikasa served as a staff officer in the Army Section of the Imperial General Headquarters in Tokyo until Japan's surrender in August 1945.

Their wedding notably occurred less than two months before the Japanese attack on Pearl Harbor which led to the United States entering World War II.

[25] On 27 February 1946, Prince Mikasa even stood up in the Privy Council and indirectly urged the Emperor to step down and accept responsibility for Japan's defeat.

General of the Army Douglas MacArthur, the most senior-ranking United States military commander in Japan at the time, insisted that Emperor Hirohito retain the throne.

[28]" Towards the end of his life, due to his advanced age, Prince Mikasa rarely made public appearances, and regularly used a wheelchair.

I'd like to spend my days pleasantly and peacefully while praying for the happiness of people around the world and thanking my wife, Yuriko, who has been supporting me for more than 70 years.

[9] On 16 May 2016, Prince Mikasa was admitted to the intensive-care unit of St. Luke's International Hospital in Tokyo's Chuo Ward, having contracted acute pneumonia.

[9] His condition eventually stabilised to the point where he began to receive rehabilitation in his bed, which included stretching his arms and legs.

Ambassador Caroline Kennedy, and former imperial family members, Sayako Kuroda (grandniece) and Noriko Senge (granddaughter) and their husbands, attended the funeral.