Mardi Gras has a number of things associated with it that can bring in large numbers of crowds every year: the floats, the throws, getting drunk in public and not getting called out as an alcoholic. But for a lot of people (myself included), the main reason they go out for Mardi Gras is to feel the tremors of thumping percussion under their feet and hear the sounds of horns shooting through the air. In other words, to see the marching bands. Being a band kid myself when I was in high school, I can tell you that there are a good number of people who go out to Mardi Gras just to see the bands (and also throw beer cans into tubas, but that's a different story). Anyway, just like the other traditions that are associated with the season, marching bands have a long-standing relationship with Carnival.
The history of Mardi Gras marching bands goes as far back as the late 1800s, after the Civil War. Not only were there military bands, but freed African Americans also began forming brass bands (which only consisted of drums and brass instruments) all around New Orleans. These bands became a vital element for an important event in the city: jazz funerals (aka second lines). These jazz funerals were marches from where a church reception was held to where a loved one would be buried. The family and friends of the deceased, along with a brass band, would march the entire route with the body. They would sing songs, play music, and celebrate the notion of the loved one's departed soul reuniting with God.
These marching processions would eventually influence the earliest forms of Mardi Gras parades, with the earliest formal Mardi Gras parades (with horse-drawn floats and the bands themselves) starting to crop up around 1857. So instead of marching to mourn and remember the dead, these bands marched to the beat of joyful celebration and excess.
This mold of Mardi Gras wouldn't really change so much in the next century or so (in regards to the bands, at least). That is, until history was made in 1967, on the 95th annual ride of the Rex parade. This is when the St. Augustine High School Marching 100 was invited to march in Rex, making it the first all African American high school band ever to do so. The Marching 100 had already been in the Zulu and Krewe of Freret parades before that, but this marked the first time an all-black marching band was featured in what is still considered by many to be the most important and well-known parade in the Mardi Gras season.
This act ended up making a large change in the face of Mardi Gras marching bands. Because of this, you're able to see a lot more bands participating in all kinds of parades that were previously unavailable to them. Nowadays, any band has the opportunity to march in any parade that they want. Now, you're able to see bands like the Edna Karr Marching Cougars, the McDonogh #35 Marching Roneagles, and the O. Perry Walker Charger Band in multiple different parades throughout the season (gotta give a quick shout-out to my old unit, the Archbishop Shaw Marching Eagles).
So, go out and enjoy yourselves this Carnival season. While you're out on the parade route, show some appreciation and give a shout-out to the kids marching in those bands. They're as much a part of Mardi Gras history as the parades themselves.