Taken from the Star Wars Wiki

Montross was a Mandalorian–turned–bounty hunter who competed with Jango Fett to become the template for the clone troopers of the Galactic Republic. Originally one of Jaster Mereel's True Mandalorians, Montross fought in the Mandalorian Civil War, where he was known for his brutal tactics and acts of recklessness. When a young boy called Jango Fett joined the Mandalorians and was treated like a son by Mereel, Montross was angered by Mereel's favoritism of Fett and became contemptuous of him. In an engagement on Korda Six, Montross left Mereel to die. He attempted to claim the title of Mandalore, but was thwarted by Fett, who told the true story of Mereel's death to the other Mandalorians, took over, and banished Montross.

Montross became a bounty hunter; known for his ruthlessness, he almost never took a live bounty. In 32 BBY, he and Fett, now also a bounty hunter, were selected by the ex-Jedi Count Dooku to participate in a hunt to capture or kill the leader of the Bando Gora cult, Dooku's former apprentice Komari Vosa. Montross did battle with Fett on Coruscant and Malastare, then later set about tracking the other bounty hunter to lead him to Vosa's lair. After killing Fett's friend and contact Rozatta, Montross learned that the Bando Gora headquarters was on the graveyard moon of Kohlma. On that world he once again fought with Fett and was defeated, before being torn to shreds by the Bando Gora.