Accounts on how Sima Yi joined the service of Cao Cao differ, but he accepted his first position in Cao's camp at the age of 30.
Under Cao Cao, Sima Yi rose through the ranks of Dongcaoyuan (東曹掾; in charge of bringing officials into service), Registrar (主簿; an administrative position), and Major (司馬; position in charge of aides and advisors).