By far the largest, most lavish Mardi Gras celebration in the US is in New Orleans, Louisiana. Mardi Gras has been celebrated in New Orleans since as early as the 1700s. Festivities included masked balls and bawdy street processions, which by 1806 had become so rowdy that they were forbidden. In 1817 it became illegal to wear masks. These laws were more or less ignored. Both the festivities and masks became legal again by 1827, when New Orleans came under American control. During the 1800s, the Carnival season was limited from January 1 until Mardi Gras as an attempt to keep the people from celebrating all year round. Later the season was lengthened to last from November 1 to June 1.

Krewes are the New Orleans clubs that host mask balls and parades during Mardi Gras. The oldest krewe is Comus, originally called “The Mystick Krewe of Comus.”
Much of the Mardi Gras krewe traditions began with the Krewe of Rex. The Krewe of Rex was formed in 1872 to welcome the Russian Grand Duke Alexis Romanoff during carnival season on his visit to New Orleans. Forty businessmen founded Rex and throw a parade in honor of the Grand Duke. The colors of the house of Romanoff--purple, green , and gold—were adopted as the official carnival colors. The Grand Duke was interested in American actress Lydia Thompson and was fond of the song “If Ever I Cease To Love.” The song was adopted as the official song of carnival.

Today, the Rex parade is the main event on Mardi Gras. The King of Rex is the King of Carnival. He is always either a civic or business leader.

Resources
The following sites and references were used when creating this page:

http://www.experienceneworleans.com/mardigras/mardi1.html
http://www.fattuesday.com
http://www.holidays.net/mardigras/
http://www.icorp.net/carnival/history.htm
http://mardigras.neworleans.com/
"Mardi Gras." Microsoft Encarta 96 Encyclopedia. 1996 ed.
McSpadden, J. Walker. The Book of Holidays. Thomas Y. Crowell Company:New York, 1935. 331-332.