On September 26, 1942, Mitsuye Endo filed a petition for a writ of habeas corpus in the United States District Court in San Francisco, arguing that the War Relocation Authority had no legal authority to detain a citizen who was loyal to the United States. She was twenty-two years old and had been incarcerated at the Tule Lake War Relocation Center in northern California for five months. She had passed a loyalty questionnaire, had a brother serving in the United States Army, worked for the California Department of Motor Vehicles before the war, and spoke no Japanese.

Endo was born in 1920 in Sacramento to Japanese immigrant parents. She was one of over 110,000 Japanese Americans — the majority of them American citizens — forcibly relocated from the West Coast in 1942 under Executive Order 9066, signed by President Franklin Roosevelt.

Her attorney James Purcell, working with the Japanese American Citizens League, selected her as the ideal test case precisely because her loyalty was unimpeachable by any government standard. The case moved slowly through the courts while the WRA continued to operate the camps. In 1944, while her case was pending before the Supreme Court, she was offered the opportunity to leave the camp and relocate to the Midwest or East — outside the restricted zone — as part of a "leave" program. She refused, not wanting to moot her case.

The Supreme Court decided Ex parte Endo on December 18, 1944, ruling unanimously that the WRA could not detain a concededly loyal citizen. The decision was timed to be released the same day the Army announced the camps were closing — minimizing political embarrassment.