It is not known when Liu Jun was born. His father Liu Chongwang was a prominent late-Tang official and served as a chancellor during the reign of Emperor Zhaozong. Several of his uncles, including Liu Chongwang's older brother Liu Chonggui, were also prominent. During Emperor Zhaozong's reign, the realm was in great turmoil. Sometime after Liu Chonggui's becoming the military governor of Qinghai Circuit, early in Emperor Zhaozong's Dashun era, Liu Jun, wanting to avoid the turmoils, followed his uncle Liu Chonggui to Qinghai and became dependent on Liu Chonggui. He remained at Qinghai's capital Guang Prefecture after Liu Chonggui's death in the middle of Emperor Zhaozong's Qianning era.
Liu Chonggui had, in or around 894, commissioned Liu Yin as the defender of Heshui Base, and then as the prefect of Feng Prefecture. Therefore, after Liu Yin subsequently became the military governor of Qinghai, first in an acting capacity in 901 and then officially in 904, Liu Yin, in showing gratitude to Liu Chonggui, treated Liu Jun with respect. He invited Liu Jun to serve on his staff, serving with Li Yinheng and Ni Shu, often consulting them on key decisions.
In 917, Liu Yin's brother and successor Liu Yan, who had previously been a vassal of Tang's successor stateLater Liang, declared himself emperor of a new state of Yue. He commissioned Liu Jun as the minister of imperial clan affairs and deputy minister of public works. Toward the end of Liu Yan's Qianheng era, there was a time when Liu Yan mobilized an army and placed it at Chao Prefecture, preparing to launch it against Southern Han's northeastern neighbor Min. Liu Jun found this inadvisable, so he discussed this with the chancellor Yang Dongqian. Yang agreed with him and tried to dissuade Liu Yan, to no avail. In 924, Liu Yan led the troops to take up a position between Min's Ting and Zhang Prefectures. A Min army attacked him, and he was forced to withdraw. He thus greatly regretted not listening to Yang and Liu Jun. After Yang died in 936, Liu Yan promoted Liu Jun to be chancellor, with the titles of Zhongshu Shilang and Tong Zhongshu Menxia Pingzhangshi, to replace Yang. It was said that Liu Jun was honest and frugal. He often encouraged Liu Yan to love his people and rest his army, in terms that were delicate but firm. He died while serving as chancellor, but the date was not given.
