[1] The founding members stated their purpose as cherishing the memories and associations of the war waged in defense of the unity and indivisibility of the Republic; the strengthening of the ties of fraternal fellowship and sympathy formed by companionship in arms; the relief of the widows and children of dead companions of the order; and the advancement of the general welfare of the soldiers and sailors of the United States.
Many people at first gave credence to these rumors, including three of the officers assigned to the honor guard for Lincoln's body as it was transported to Springfield, Illinois, for burial; these three men, Brevet Lt. Col. Samuel Brown Wylie Mitchell, Lt. Col. Thomas Ellwood Zell, and Captain Peter Dirck Keyser, are considered the founders of the Order.
At its height about 1900, the Order had more than 8,000 Civil War veterans as active members, including nearly all notable general and flag officers and several presidents: Ulysses S. Grant, William T. Sherman, Philip H. Sheridan, George H. Thomas, George B. McClellan, Rutherford B. Hayes, Chester A. Arthur, Benjamin Harrison, and William McKinley.
The Order's fame was significant enough to inspire John Philip Sousa to compose the "Loyal Legion March" in its honor in 1890.
Predecessors to it include the Society of the Cincinnati, instituted in 1783, and the Aztec Club of 1847, both similarly founded by and for commissioned officers of the United States military.
Among other activities, Companions organize and participate in commemorative events, provide awards to deserving ROTC cadets, and assist with preservation efforts.
Current national officers for the 2023–2025 term include Commander-in-Chief Michael Timothy Bates, Esq, of New Jersey; Senior Vice-Commander-in-Chief Paul Davis of Michigan; Junior Vice-Commander-in-Chief William F. Forbes of Pennsylvania; Treasurer-in-Chief Lee Alan Tryon, CPA, of Connecticut; Recorder-in-Chief Gary L. Grove, PhD.
Note – Presidents Andrew Johnson and James Garfield were both generals in the Union Army during the Civil War and were thus eligible to be veteran companions of MOLLUS but did not join the Order.
From 1865 to 1890, a limited number of civilians who contributed outstanding service to the United States during the Civil War were elected into the Order as 3rd Class Companions.
Anderson Cooper is also eligible for hereditary membership in MOLLUS because of his descent from Major General Hugh Judson Kilpatrick.
Secretary of State John Foster Dulles and his brother, CIA Director Allen Dulles, were eligible for membership in MOLLUS by right of their descent from their maternal grandfather Colonel John W. Foster, who served as Secretary of State in the administration of President Benjamin Harrison.
King Manuel II of Portugal (1889–1932) was eligible to become a hereditary companion of MOLLUS as his mother was a daughter of Philippe d'Orleans.
Prince Amedeo, Duke of Aosta (1943–2021), head of the House of Savoy and claimant to the throne of Italy, was eligible for hereditary MOLLUS membership but was elected as an honorary member instead.
Prince Pierre, Duke of Penthièvre was a cousin of the Count of Paris and served in the Union Navy as an ensign on the frigate USS John Adams.