The division headquarters is a unit of the New York Army National Guard. The division currently includes Army National Guard units from fourteen different states, including Connecticut, Maine, Maryland, Massachusetts, New Hampshire, New Jersey, New York, Rhode Island, and Vermont. As of 2007, 67 percent of 42ID soldiers are located in New York and New Jersey.
The 42ID came to be known as the "Rainbow Division". When the United States declared war on Germany in 1917, it federalized National Guard divisions to quickly build up an Army. In addition, Douglas MacArthur, then a major, suggested to William A. Mann, the head of the Militia Bureau, that he form another division from the non-divisional units of several states. Secretary of War Newton D. Baker approved the proposal, and recalled Douglas MacArthur saying that such an organization would "stretch over the whole country like a rainbow." The division was created using units from 26 states and the District of Columbia. The name stuck, and MacArthur was promoted to colonel as the division chief of staff.