Prior to the advent of Comus, Carnival celebrations in New Orleans were mostly confined to the Roman Catholic Creole community, and parades were irregular and often very informally organized. In December of 1856 a number of New Orleans businessmen, mostly uptown Protestant Americans from other parts of the United States, gathered to found the organization to produce a parade and ball on Mardi Gras night. The inspiration for the name came from John Milton's Comus. Part of the inspiration for the parade was a Carnival group in Mobile, Alabama called the Cowbellions.
The first Comus parade was held on Mardi Gras 1857, and this became an annual event. Other organizations sprung up in New Orleans in the 19th century inspired by the Comus model and also came to be known as "Krewes".
Members of Comus are always masked at when appearing in public at Krewe events, and their identies is supposed to remain unknown. Membership in Comus has historically been identical or nearly identical to membership in the private New Orleans men's club The Pickwick Club.
Parading on Mardi Gras night, Comus was the final parade of the New Orleans carnival season. It was much smaller than both the more modern parades and it's fellow 19th century Mardi Gras day parade Rex. (Comus also did not stage parades for a number of years, so that by the late 20th century Rex, although 16 years younger than Comus, had held more parades.) The Comus parades became known for their sometimes obscure themes relating to ancient history and mythology. While other New Orleans parades might have themes like "Foods of the World" or "Broadway Show Tunes", Comus would present themes like "Serpent Deities of the Ancient Near East".
In 1991 the New Orleans city council passed an ordinance that city funds could not be used to pay for police and sanitation for any event held on public streets by any organization which was racially segregated according to its own bylaws. The Comus organization (along with Momus, another 19th century Krewe) decided to no longer parade rather than to either change their bylaws or pay for the municipal expenses of their parade. The Mystick Krewe of Comus still holds an annual ball on Mardi Gras night.