It is composed of spouses, mothers, fathers, daughters, sons, granddaughters, grandsons, and brothers, & sisters of American war veterans.

A group of 20 officers who served in the American Expeditionary Forces in World War I were asked to suggest ideas on how to improve troop morale.

After two planning caucuses held by a committee of officers who had the confidence and respect of their military comrades, they designed a constitution to govern the group and set up headquarters in New York City to begin work on its programs of relief, employment and Americanism.

The committee decided to build a new organization from the ground up, so the Auxiliary could then carry forward the phases of Legion activities more suitably performed by women.

In 2019, the American Legion's National Convention voted to replace the word "wife" with "spouse" in the organization's constitution and bylaws section regarding eligibility to be a member of the American Legion Auxiliary; since then, male and female spouses of U.S. veterans have been eligible.