Sima Zhong was born to Sima Yan and his wife Yang Yan in 259 AD, while Sima Yan was still the assistant to his father, the Cao Wei regent Sima Zhao. He was their second son, but after the early death of his older brother Sima Gui (司馬軌), he became the oldest surviving son. It is not known when his developmental disabilities became apparent, but, in any case, after Sima Zhao died in September 265, and Sima Yan subsequently forced the Cao Wei emperor Cao Huan to abdicate to him in February 266 (which ended Cao Wei's reign and started Jin's as Emperor Wu), he made the seven-year-old Prince Zhong crown prince in 267 AD.