It is not known when Hu Jinsi was born (other than that it would have been during the late Tang years), but it is known that he was from Hu Prefecture (湖州, in modern Huzhou, Zhejiang). When he was young, he was a cattle butcher. He later became a soldier in the army of Zhenhai Circuit (鎮海, headquartered in modern Hangzhou, Zhejiang), which was then ruled by its military governor (Jiedushi), the late-Tang warlord Qian Liu (who would later be Wuyue's founding king, as King Wusu).
In 902, Qian Liu came under the attack of Tian Jun the military governor of Ningguo Circuit (寧國, headquartered in modern Xuancheng, Anhui), a vassal of the major warlord Yang Xingmi the military governor of Huainan Circuit (淮南, headquartered in modern Yangzhou, Jiangsu), and his headquarters at Hang Prefecture (杭州) came under Tian's siege. While Yang, not wanting to see Tian becoming overly powerful, ordered Tian to withdraw, Tian extracted the price of having Qian send a son to return with him to Ningguo to serve as hostage (with Tian agreeing to give a daughter to him in marriage). Qian's son Qian Chuanguan volunteered, even though the situation was considered extremely perilous. Hu and another soldier, Dai Yun (戴惲), served as Qian Chuanguan's attendants on this journey, putting themselves in the same risks that Qian Chuanguan was under. After Tian rebelled against Yang and was killed in 903, Qian Chuanguan returned to Qian Liu's domain; presumably, Hu (and Dai) also accompanied him in his return.
After Qian Chuanguan (whose name had been changed to Qian Yuanguan by that point) assumed the reins of the Wuyue state (and would later take the title of King of Wuyue, a title that Qian Liu had, as King Wenmu) after Qian Liu's death in 932, he, remembering Hu's faithfulness, made Hu a general. Hu was eventually promoted to the post of right commander of the headquarter corps (右統軍使, You Tongjunshi).