History
Just why there is a Mardi Gras is no mystery. When Louis XIV permitted Carnival celebrations (another term for Mardi Gras) little did he know that his edict would last for hundreds of years.
In the early days Mardi Gras was celebrated with balls and masquerades. Later the Creoles would celebrate it by observing Shrove Tuesday (the actual Mardi Gras day) as a gala day ending with a masked Mardi Gras ball.
Still other early celebrations included public fairs. Gambling houses, dance halls and similar places provided other types of celebrations. Later Quadroon Balls were the fad for years. The aristocratic Creoles sponsored Bals de Rois which were limited to members of the inner circles of the old city.
During the Spanish era, masking was discretionary. This stopped soon thereafter as public disturbances became prevalent. Masking was later prohibited under Spanish rule.
During the early American territorial times, Mardi Gras was restricted to dancing only. Masks were undoubtedly worn at private celebrations. When masking was attempted at public balls, it was abruptly stopped. As time progressed under early American rule, celebrations took place in different areas in different ways. Some legal; some illegal.
Masking
was prohibited after the Louisiana Purchase for two decades. Public ballrooms
and private assemblies still saw celebrations. Dancing began to take on a
greater importance. Other celebrations included balls, concerts and dramatic
performances. Two theatres, the Theatre d'Orleans and the Salle d'Orleans
were home to ballrooms where vast dancing floors hosted hundreds of dancers.
In 1824, Carnival began to take on a new look. A public hall began to advertise the society balls with no mention of the possible enforcement of anti-masking laws. A half-dozen exclusive society balls were held on Fridays. Four select masked balls characterized the final nights of Mardi Gras then. Two hundred couple were invited by subscription and obtained costumes from the property rooms of the theatre itself.
The first organized parade in New Orleans was on Mardi Gras day in 1837. Little is known of it, but it may have been called the "Cowbellions." Costumes were flashy and gruesome, and the parade was noisy. Urchins, vagrants and other onlookers were the norm. After the parade was finished the costumers went to the customary balls at different locations.
The earliest description of a Mardi Gras float or vehicle was in 1839.
A giant fighting cock, six feet tall, drawn by a carriage by two horses with
flapping wings. It crowed to the crowd. The news media reported that it was
rumored that male maskers impersonated women and women rode through the crowds
tossing favors from carriages. 
The history of Carnival is more vivid and documented in the mid-nineteenth century. The Mistick Krewe of Comus arrived in 1857 and helped usher in what the modern day celebration is really like.
The mythological son of Bacchus and Circe, Comus was the name chosen for the first parade organization. It was a secret society with an initial membership of six men. They met at the old Gem Restaurant with thirteen others and the movement was approved by all. The number soon tripled and the official name was chosen. A solemn oath of silence was taken by everybody which stil exists today. Costumes were obtained from Mobile and a secret invitation group put out three thousand invitations to the elite of Louisiana and nearby states.
On February 24, 1857, krewe members put their costumes on and formed a line of march. His Satanic Majesty followed them on a second float. Comus was on the first float. After the parade, the krewe retired to a nearby banquet hall for wine and enjoyment. This continued till the next morning.
In 1861, Comus member walked in a torchlight procession. The Civil War in the subsequent years would see the celebration cancelled. Most able-bodied men wer off to war. The celebration returned in 1866 with ONE float. Later celebrations of Comus saw it become foot parades and float celebrations in different years. It should be noted that in 1870, Comus maskers were ordered to have at least one member not masked.
In 1870, another group appeared on the scene. The Twelfth Night Revelers put on a series of torchlight processions and tableau balls then. In 1872, the Grand Duke Alexis of Russia's visit to New Orleans enticed another group to form a Carnival organization, Rex.
The Knights of Momus appeared in 1872 also. Their floats were illuminated with flambeaux. Proteus arrived in 1883, the Atlanteans in 1891, the Elves of Oberon in 1895, and Nereus in 1896. Others followed throughout the decades since and the celebration remains strong and healthy.