[2] While he maintained his own state, he also nominally served as a vassal of the Later Qin, Eastern Jin, and Northern Wei dynasties.
Initially, he was defeated by Lü Guang's son Lü Zuan and fled into the mountains, but he was soon joined in rebellion by his cousin Juqu Nancheng (沮渠男成), who sieged Jiankang (also in modern Zhangye) and persuaded Duan Ye the governor of Jiankang Commandery to accept leadership of the rebels, establishing Northern Liang.
Juqu Mengxun, having taken the ducal title, promoted a number of officials who were considered capable, and it was said that the people of his state were pleased.
He also nominally submitted to the Later Qin emperor Yao Xing as a vassal, although remaining in reality independent.
He became fearful, and he sent his brother Juqu Ru (沮渠挐) the Marquess of Dugu and official Zhang Qian (張潜) to meet Yao Xing's uncle Yao Shuode (姚碩德), who had just recently sieged Later Liang's capital Guzang (姑臧, in modern Wuwei, Gansu) and forced the Later Liang emperor Lü Long to submit, offering to surrender his state to Later Qin.
Yao Shuode was pleased, but upon return to Northern Liang, while Zhang recommended such surrender, Juqu Ru argued against it, and Juqu Mengxun, while remaining nominally a Later Qin vassal, executed Zhang and never actually surrendered his state.
After initially refusing, Juqu Mengxun gave in to Southern Liang demands after Tufa Lilugu defeated him in battle.
In 402, with Guzang under a severe famine, Juqu Mengxun attacked Later Liang, causing Lü Long to seek aid from Southern Liang, but before Southern Liang forces could arrive, Lü Long defeated Juqu Mengxun, and Juqu Mengxun made peace with Lü Long, offering him food for famine relief.
In fall 407, Tufa Rutan made another attack on Northern Liang, but Juqu Mengxun was able to defeat him.
The people of Guzang, because Tufa Rutan had previously carried out massive executions after a failed rebellion, collapsed in fear, and more than 10,000 households surrendered to Northern Liang.
Tufa Rutan, apprehensive of both Juqu Mengxun and a rebellion by Zhequ Qizhen (折屈奇鎮) in the south, made peace with Juqu Mengxun and moved his capital back south to Ledu (樂都, in modern Haidong Prefecture, Qinghai).
In winter 412, Juqu Mengxun moved the capital from Zhangye to Guzang, and he claimed the greater title of Prince of Hexi.
In the summer of that year, he repelled another attack from Tufa Rutan, and then again put Ledu under siege for 20 days, but could not capture it.
Also in 413, while Juqu Mengxun was sleeping, his eunuch Wang Huaizu (王懷祖) tried to assassinate him, but only hurt his foot.
In 418, Juqu Mengxun made an attack on Western Liang, but Li Xin refused to engage him, and he withdrew.
Li Xin, believing wrongly that Juqu Mengxun's defenses were down, decided to attack Zhangye, against the advice of Song Yao and Zhang Tishun, as well as his mother Princess Dowager Yin.
His generals then advised him to quickly withdraw to Jiuquan, but Li Xin, stating that he had disobeyed his mother and would only be able to see her again after a victory, engaged Juqu Mengxun again, suffering an even greater defeat, and he was killed in battle.
Li Xun's official Song Cheng (宋承) rebelled and offered the city to him, and Li Xun committed suicide, ending Western Liang; contrary to the pacification policy he carried out at Jiuquan, Juqu Mengxun slaughtered the populace of Dunhuang.
Juqu Mengxun sent messengers to persuade the Xia emperor Helian Chang (Helian Bobo's son and successor) to make a surprise attack on the Western Qin capital Fuhan (枹罕, in modern Linxia, Gansu).
Helian Chang, in response, sent his general Hulu Gu (呼盧古) to attack Wanchuan and Wei Fa (韋伐) to attack Nan'an (南安, in modern Dingxi, Gansu), and while Western Qin was able to hold Wanchuan, Nan'an fell, at great loss.
In winter 426, Xia forces commanded by Hulu and Wei attack Fuhan, forcing Qifu Gangui to move the capital to Dinglian (定連, also in Linxia), and Hulu and Wei then captured another important Western Qin city, Xiping (西平, in modern Xining, Qinghai), and while they then withdrew, Western Qin had been dealt a major blow.
Later that year, with Emperor Taiwu of Northern Wei having in turn defeated Helian Chang in battle, capturing Chang'an and nearly capturing the Xia capital Tongwan (統萬, in modern Yulin, Shaanxi) as well, Juqu Mengxun sent messengers to Northern Wei offering to submit as a vassal.
In 431, with Xia's emperor Helian Ding (Helian Chang's brother and successor after Helian Chang was captured by Northern Wei in 428) having first destroyed Western Qin and killed Qifu Mumo and then having been defeated and captured by Murong Mugui), Juqu Mengxun, now with his territory directly in contact with Northern Wei, sent his son Juqu Anzhou to Northern Wei as a hostage to show his loyalty.