Following the assassination of President Abraham Lincoln in 1865, rumors spread that the assassination had been part of a wider conspiracy to destroy the federal government by assassinating more of its leaders. In response, a group of officers who had fought in the Union Army formed the Loyal Legion as a society of officers who, having proven their loyalty in battle, pledged to defend the country. Patterned after the similar Society of Cincinnati that had been established after the Revolutionary War, it was organized largely during the same meetings that planned Lincoln's funeral, culminating in a meeting on May 31, 1865 in Philadelphia's Independence Hall at which the name was chosen, and a mass meeting of Philadelphia war veterans on April 20.
At its height at the very end of the 19th century, the order had over 8,000 Civil War veterans as members, including nearly all notable officers and several future presidents—Ulysses S. Grant, William T. Sherman, Philip H. Sheridan, George B. McClellan, Rutherford B. Hayes, Chester A. Arthur, Benjamin Harrison, and William McKinley, among others. The Order's fame was great enough to inspire John Philip Sousa to compose the Loyal Legion March in its honor in 1890.
As the Civil War veterans aged and died, the Order has opened hereditary membership to descendants of the original members, and today serves as more of an honorary society than a functioning military order.