The Mandalorians originated as the Taung species native to Coruscant, fighting against the thirteen human nations of that world. Eventually the race that would become the Mandalorians was driven off Coruscant and eventually conquered a world they would call Mandalore after their leader Mand'alor. They would eventually conquer the worlds of Ordo, Gargun and Shogun. Before 4,000 BBY the Mandalorians launched a raid against the planets Iskadrell and Basilisk, on Iskadrell the Mandalorians freed the slaves of that world and absorbed them into their culture. On Basilisk they converted the native population into beasts of war and took their War Droids to serve as battle mounts. The Mandalorians would later come into contact with the Sith, allying with them in a war against the Republic, eventually leading to their defeat.
The Mandalorian Wars
Several decades after their defeat the Mandalorian Neo-Crusaders began raid systems on the borders of Republic space, killing, enslaving or conscripting the native populace. After amassing a territory larger than that of the Hutts, the Mandalorians launched attacks into the Republic, quickly capturing the worlds of Onderon, Taris and Vanquo. During the Battle of Serroco the Republic underestimated the Mandalorians and constructed their defenses within the planets cities, thinking that the Mandalorians would try to avoid harming the native populace.
The Mandalorians showed their true ruthlessness by bombarding the surface cities with nuclear weapons from orbit. Later the Mandalorians were faced with something they hadn't encounter so far during their successful campaign, Jedi Commanders leading Republic troops, Jedi led by a Jedi Knight named Revan... With the arrival of Revan the tide of the war changed quickly against the Mandalorians, eventually pushing them back to the fortified moon of Dxun, which served as the Mandalorian fleet's command center, after months of fighting and losing thousands of troops and equipment to Mandalorians dug into the jungle, booby traps and the native fauna, the Republic finally captured Dxun. The Mandalorian fleet was then lured to the taboo world of Malachor V where both the Mandalorian and the Republic Fleet were nearly eradicated when the Jedi that would become known as the Jedi Exile activated the Mass Shadow Generator. This event crushed the Mandalorians dreams of conquering the Republic and they soon surrendered.
After their defeat in the Mandalorian Wars the Mandalorian clans scattered and many became mercenaries and bounty hunters. Years later, a new Mand'alor would arise and attempt to reunite the Mandalorians under Clan Ordo on the forest moon of Dxun. While many Mandalorians joined with this new Mand'alor most were skeptical and remained as mercenaries or bounty hunters.
Rise of the True Mandalorians
Jaster Mereel, a journeyman protector from the world Concord Dawn was given the title of Mand'alor some time before 60 BBY. He established what would be known as the Supercommando Codex, which
he hoped would restore honor to the Mandalorian people, describing them as highly paid soldiers, as opposed to raiders and butchers. This led to a split between the Mandalorians when several clans disagreed with the Codex. Jaster's faction, the True Mandalorians fought a large splinter group known as the Death Watch, the War eventually led Jaster and some of his men to Concord Dawn where they were hunted by Death Watch soldiers. They eventually sought refuge on the farm of the Fett family. As the Death Watch murdered the members of the Fett family Jaster escaped with their young son, Jango. Jaster would lead the True Mandalorians until 52 BBY, when he was killed by the leader of the Death Watch, Vizsla on Korda 6. Jango Fett would become the next Mand'alor which would have a tremendous impact on Mandalorian society, After several years serving as mercenary soldiers, the True Mandalorians excepted a job on the world of Galidraan to eliminate political dissidents. The entire operation was a trap set up by the Death Watch and the Ruler of Galidraan contacted the Jedi Order for assistance, telling them the Mandalorians were killing "political activists". A large Jedi task force soon arrived and engaged the Mandalorians in combat. Jango Fett was one of the two survivors of the original 300 True Mandalorians. Eleven Jedi were killed in the Battle, including seven Fett killed with his bare hands. Jango was sold into slavery by the ruler of Galidraan, he eventually escaped and reclaimed his armor, he then went on to eradicate the Death Watch aboard their ship and defeat Vizsla in single combat, Vizsla was promptly eaten by local fauna, which left Jango alone because he was poisoned.
Boba Fett, the clone son of Jango Fett would later become the new Mand'alor, leading the Mandalorian Protectors during the Yuuzhan Vong Invasion. Fett tricked the Vong into think of him as an ally while he gathered intel and samples on the Yuuzhan Vong to give the the Galactic Federation of Free Alliances. He eventually betrayed the Vong and joined the fight against them.