The True Mandalorians were a faction of Mandalorians led by Jaster Mereel that wanted to move past the crusading ways of old and become mercenaries. Due to differing beliefs, the Death Watch (led by Tor Vizsla) opposed the True Mandalorians' ideals.
While fighting in this civil war (the first Mandalorian Civil War), the True Mandalorians exercised their beliefs by becoming soldiers for hire in the galaxy. They fought the battles for governors and kings and were paid well for their expert work.
The True Mandalorians followed the Supercommando Codex created by Jaster Mereel. It was a guideline for all Mandalorians to follow regarding their actions. Instead of being blood thirsty barbarians, they were to be honorable soldiers.
Their leadership was composed of:
Jaster Mereel (Mand'alor)
Montross (Second in command)
Jango Fett (Third in command, Jaster's heir)
Jaster Mereel (Mand'alor)
Montross (Second in command)
Jango Fett (Third in command, Jaster's heir)
While in a battle on Korda 6, Jaster Mereel was set up by Montross and was killed by the Death Watch. Montross was kicked out of the group and Jango became the new Mand'alor.
Jango would lead the True Mandalorians for a few more years until the Battle of Galidraan. They were working for the governor of the planet to put down a rebellion.
However, the Death Watch were secretly working with the Governor and set the True Mandalorians up. They contacted the Jedi and told them that the True Mandalorians were slaughtering innocent people on Galidraan. The Jedi intervened and a fight ensued.
The result of this battle was the end of the True Mandalorians. The only Mandalorian survivor of this battle was Jango Fett. Fett slew many Jedi with his bare hands before being defeated. He later singlehandedly destroyed the Death Watch and killed Tor Vizsla in single combat.
Even though the True Mandalorians were completely dismantled, their beliefs would triumph over the Death Watch and the Supercommando Codex would be followed by most Mandalorians. The Journeyman Protectors even based their organization after the True Mandalorians.