Mongke once served under GeneralIroh in the Fire Nation Army, before becoming the leader of the Rough Rhinos. In 91 AG, he and the group were responsible for the destruction of Jet's hometown and the death of his parents. In late 99 AG, they invaded another village at the western Earth Kingdom coast, forcing the residents to become refugees, including Tahn and Ying.
In spring 100 AG, the Rough Rhinos had their first encounter with Team Avatar. Colonel Mongke and his men tried to raid Team Avatar's campsite, though they failed to kidnap the Avatar and were left with only Sokka's boomerang as a valuable loot.
The evening of the following day, Mongke led his Rough Rhinos into Chin Village, claiming the Earth Kingdom town in the name of Fire Lord Ozai. Mongke demanded to know the identity of the town's leader in order to dethrone him, though when Aang managed to defeat Kahchi, the colonel led his remaining team into the town and started to burn it down. He encountered Aang in one of the village's narrow allies, where they faced off against each other. Although he managed to separate Aang from Kyoshi's metal fans that he was using to augment his airbending, Mongke was eventually defeated when Aang blasted him through a wooden fence.
Later that spring, Mongke learned that the Avatar had been spotted in a town a few miles from their current location and subsequently led his men there in order to apprehend the Avatar. When they arrived, they saw Sokka seemingly bending water and earth, mistaking him to be the Avatar. After Kahchi captured Sokka, Mongke tried to fend off Katara, though his flames were easily overpowered by her waterbending. He arrogantly claimed that even the combined effort of Aang's airbending, Katara's waterbending, and Toph's earthbending would be no match for a firebender of his level. However, before he could even finish his sentence, the three young benders unleashed their attacks on him and the other Rough Rhinos, expelling them from the town.
Mongke and his men later tracked down and ambushed Iroh and PrinceZuko near the Si Wong Desert, seeking to apprehend them for their traitorous actions. He attacked Iroh with his firebending, though the old general easily brushed his fire balls aside. Mongke was taken out by Zuko who had used the colonel's distraction to sneak up behind the Rough Rhinos' leader and kick him off his mount.
Life as a mercenary
After the end of the Hundred Year War, Mongke and his men offered their services for hire to anyone, eventually coming under the employ of Loban and Lao Beifong as their personal security detail. In 102 AG, the Rough Rhinos accompanied their employers to Yu Dao, but upon returning to the Earthen Fire Refinery, Mongke spotted Avatar Aang and proceeded to rudely greet him, referring to him as an unwanted person.
Mongke was pleased to be ordered to escort Team Avatar off the refinery's premises, giving Aang the choice to come quietly or with a struggle. When they refused to come, Mongke was quick to instigate a fight with them. He was instantly pushed back, though, when Toph raised the earth under his feet and Aang used his airbending to blow him clear of the factory's buildings. He recovered and targeted Sokka with his firebending. When the Southern Water Tribe warrior came at him with his machete, Mongke easily caught the blow on his chest plate, taunting Sokka that he had used chopsticks bigger than the young warrior's arms. When Sokka ran, Mongke whistled for his komodo dragon and gave chase, along with Yeh-Lu and a hammer-wielding member of the Rough Rhinos. They were tricked, however, to run down the refinery's fence and resume the battle near a river, which was in Katara's advantage who had joined the skirmish. Although Mongke was convinced he could beat her, he and his two companions ended up being encased in ice, unable to move anything but their heads. As they were left there, a shivering Mongke was told by Yeh-Lu that he should have incorporate more armor in his uniform, as that would have prevented the ice from undercooling him.
Colonel Mongke was a capable firebender and a tough opponent to defeat, especially when fighting alongside the other Rough Rhinos. He was a good tactician and had a knack for singling out weaker Earth Kingdom towns. At one point, he nearly captured the Avatar. Mongke and the rest of the Rough Rhinos were also said to be capable singers.
His name seems to reflect a Mongolian influence. "Mongke" is the name of Genghis Khan's grandson and means "eternal", which makes sense given that Khan himself has been seen as a destructive warmonger.