He hailed from Chenliuyu 陳留圉 (modern Qixian 杞縣, Henan) and was famous for his vast knowledge in literature, history and science that he had acquired during his youth. He therefore entered the service of Grand Mentor (taifu 太傅) Hu Guang 胡廣 and was appointed gentleman of the interior (langzhong 郎中) as editing clerk (jiaoshu 校書) in the Dongguan Archives 東觀, and later promoted to court gentleman for consultation (yilang 議郎).
A memorial by him criticising the court caused his dismission. He also suffered during the factional strifes between the court officials and the eunuchs and lived for a long time a private life.
The mighty warlord Dong Zhuo 董卓 appointed him attendant censor (shiyushi 侍御史) and later Leader of the court gentlemen (zhonglangjiang 中郎將). When Dong Zhuo was killed, Cai Yong was arrested by Wang Yun 王允 and put into jail.
During his time at the court of the Han dynasty he was one of the promoters of China's first project of incising the Confucian Classics into stone slabs, the so-called Xiping shijing 熹平石經 "Stone Classics from the Xiping reign (172-177 CE)". He also wrote the calligraphy for this project. One aim of this project was to propagate the correct wording of the Classics. Emperor Ling 漢靈帝 (r. 167-189) therefore ordered Cai Yong to determine the correct words and characters and to write it in the official chancery script (litizi 隸體字) on the slabs. Chisellers then cut the characters into the surface of the stone. It was finished in 183 CE, and the slabs were erected in the court of the National University (taixue 太學). All scholars were allowed to take rubbings from the Stone Classics, and disputes about the correct text of the Classics were ended. A situation as before, when scholars wrote their versions in lacquer on the Lantai Pavillion 蘭臺, could be avoided. On the other hand, the creation of an orthodox text also contributed to the extinction of different versions of the Classics.
Cai Yong is also known as a famous writer and composer of a lot of poems and rhapsodies, like the Shuxing fu 述行賦 that laments about the contemporary luxury at the court and among the nobility, in contrast to the sad life of the average population. Some prose texts of Cai Yong were written on steles. He was famous for his excellent calligraphy that served as a model for later students learning to write seal script (zhuanshu 篆書) and chancery script.
Cai Yong's collected writings were published as Cai Zhonglang ji 蔡中郎集. They are lost, and only some fragments could be recovered as quotation in other books.