[3] Mitsui emigrated to Canada in 1908, where he worked at first as a waiter at the Union Club in Victoria, British Columbia.

The federal government was reluctant to accept recruits from ethnic minorities; the Cabinet of Canada rejected a battalion of 171 volunteers the Canadian Japanese Association had trained in early 1916, in which Mitsui had taken part.

[4] Strong feelings against Asian immigrants in British Columbia led to widespread discrimination, and even Anti-Oriental Riots of 1907.

[14] Mitsui and other members travelled to Victoria to promote the Provincial Elections Act, which would extend suffrage to Japanese-Canadian war veterans in British Columbia.

Their campaigning led to the passage of the bill by a single vote in the Legislative Assembly of British Columbia.

Mitsui wrote the Minister of National Defence on behalf of Japanese-Canadian veterans pledging "their unflinching loyalty to Canada as they did in the Last Great War".

[16] Nevertheless, existing anti-Japanese discrimination only increased as Japan became a war enemy, exacerbated by reports of the brutality of the Japanese forces in Hong Kong.

A series of Orders in Council in 1942 deprived Canadians of Japanese descent of their property and rights, culminating in Japanese-Canadian internment for the duration of the war.

[3] In 1946 Mitsui and the other remaining 33 Japanese-Canadian veterans of World War I petitioned the Canadian government to restore their civil rights, but the National Emergency Transitional Powers Act passed in 1945 only increased the restrictions.

Interned Japanese Canadians were given the option of repatriation to Japan or relocation east of the Rocky Mountains; Mitsui opted to move with his family to Southern Ontario.

[19] On Remembrance Day each year Mitsui dressed himself in his uniform and medals and stayed at home, refusing to take part in public services.

[21] Mitsui and his wife Sugiko had four children: two girls, Lucy and Amy; and two boys, George (the eldest) and Harry (the youngest).

[10] Mitsui bequeathed his medals to his grandson David,[3] who wore them to an exhibit dedicated to the Japanese-Canadian veterans of World War I at the Calgary Highlanders Museum on 12 March 1994.