Mukai is best known for his involvement in the Nanking Massacre (1937–1938), where he was implicated in atrocities against Chinese civilians and prisoners of war.

He joined the Imperial Japanese Army in the early 1930s and was eventually promoted to the rank of Second Lieutenant.

[2] During the Nanking Massacre, which lasted from December 1937 to January 1938, Mukai played a role in atrocities committed by the Japanese military.

Although some details of his specific involvement remain under investigation, his name has been cited in survivor testimonies and tribunal records.

The Nanking Massacre remains a highly sensitive and controversial topic in China–Japan relations, with ongoing debates over historical memory and justice.