He hailed from Chen 陳 (modern Huaiyang 淮陽, Henan) and was famous along the Yangtze river as a so-called wandering knight (youxia 遊俠). During the reign of Emperor Jing 漢景帝 (r. 157-141 BCE) he became a secretary (sheren 舍人) of the Heir Apparent and rose to the post of governor (taishou 太守) of the commandery of Jinan 濟南. He also was counsellor to the Prince of Jiangdu 江都 and later became Right Chamberlain for the Capital (you neishi 右內史) and Chamberlain for the National Treasury (da sinong 大司農).
Zheng Dangshi was honest and incorruptible in his office and treated the rich and and the poor equally fair. He made use of competent advisors. As an adherent of the Huang-Lao school of thought 黃老 he preferred a non-active, benevolent and peaceful government. He died as governor of Runan 汝南.