Founded on May 15, 1865 in Philadelphia, Pennsylvania by three Union officers: Brevet Lieutenant Colonel Samuel Brown Wylie Mitchell, Lieutenant Colonel Thomas Ellwood Zell, and Captain Peter Dirck Keyser. Originally, membership was restricted to Union officers only. It was orgainized into "commanderies"; a national commandery was founded in 1885, with Winfield Scott Hancock as commander in chief. The MOLLUS War Library and Museum was founded in Philadelphia in 1888.
Meetings of the state commanderies usually included a speech concernin a first hand account of the war; these speeches were later collected in published volumes by state, ofter under the title War Papers. A total of thirty-seven volumes were published between 1886 (startin with Michian) and 1912 (Pennsylvania).