Qian Hongzong was probably born in 928. His father was Qian Chuanguan (later known as Qian Yuanguan, King Wenmu), who, as of 928, had been designated the heir to his father (Qian Hongzong's grandfather) Qian Liu (King Wusu), the founding king of Wuyue. His mother was Qian Chuanguan's concubine Lady Fu, who had previously given birth to Qian Chuanguan's oldest biological son (fifth son overall) Qian Hongzun. Qian Hongzong was Qian Chuanguan's seventh son (third biological). When Qian Hongzong was born, Qian Chuanguan dreamed of a man presenting him with a box of gold, and therefore nicknamed Qian Hongzong Wanjin ("10,000 catties of gold").
Early in his career, Qian Hongzong served as an officer of the Wuyue royal guard corps, and carried the honorary title of acting Sikong (司空, one of the Three Excellencies). In 944, during the reign of his older brother Qian Hongzuo (King Zhongxian), who had succeeded Qian Yuanguan, he was sent to Yue Prefecture (越州, in modern Shaoxing, Zhejiang) to serve as the comforter of the eastern headquarters (i.e., Yue), and was given the honorary title of acting Taiwei (太尉, also one of the Three Excellencies). In 947, Qian Hongzuo recalled him to Wuyue's capital Qiantang to serve as chancellor. Later in the year, Qian Hongzuo died, and, in his will, named Qian Hongzong the military governor (Jiedushi) of Wuyue's two main circuits, Zhenhai (鎮海, headquartered at Qiantang) and Zhendong (鎮東, headquartered at Yue), as well as the honorary chancellor title of Shizhong (侍中). Shortly after, Qian Hongzong took the throne. For the time being, he used the Huitong era name of the Khitan Liao dynasty, as Qian Hongzuo had previously submitted to Liao as a vassal.