On March 1, 1921, Idaho amended its anti-miscegenation law to include additional restrictions on interracial marriage. Idaho passed its first anti-miscegenation law in 1864, which banned marriage between a white person and "any person of African descent, Indian or Chinese." The punishment for marrying in violation of the statute was imprisonment for up to two years. Idaho also passed a law banning interracial cohabitation in 1864, violation of which could result in a $100-$500 fine, six to 12 months in jail, or both. The anti-miscegenation law was amended in 1867 to increase the range of fines and the maximum possible prison time to 10 years.

The 1921 amendment to the law banned marriage between white people and "mongolians, negroes, or mulattoes," although the state's population at the time was less than .02% African American. The Idaho state legislature repealed the anti-miscegenation law in 1959.

Idaho was not unique in its attempts to obstruct marriage between the races. In the 1920s, Social Darwinism had captured the attention of the country's elite, who became concerned with maintaining and promoting the eugenic racial purity of the white race by controlling procreation. Concerned that states were not adequately enforcing their anti-miscegenation laws, eugenicists pushed for stronger measures against racial mixing and stricter classifications to determine who qualified as white when seeking a marriage license. Like Idaho, many states added the racial category "mongolian" during this time in response to an influx of Japanese immigrants to the U.S.