During Cao Cao's lifetime, he valued Sima Yi and twice invited him to serve as an official at court. The first time, Sima Yi feigned illness and refused. Cao Cao suspected he wasn't truly sick and secretly sent someone to investigate. Sima Yi handled the situation in time and managed not to be exposed. Knowing that Cao Cao would not let the matter rest and that pretending to be ill couldn't last a lifetime, Sima Yi waited for a while, then publicly announced that he had recovered from his illness and accepted Cao Cao's invitation to enter court service.
Cao Cao promoted Sima Yi several times, eventually appointing him as the Crown Prince's Attendant, tasked specifically with assisting Cao Pi. Cao Pi placed great trust in Sima Yi, and Cao Cao frequently consulted him on state affairs, often adopting his strategies. Before long, Sima Yi was promoted to Chief Military Advisor to the Chancellor due to his outstanding achievements.
After Cao Cao's death, Sima Yi managed the funeral arrangements with such meticulous organization that Cao Pi was deeply moved. When Cao Pi later succeeded to the throne as King of Wei, he immediately promoted Sima Yi to Marquis of Hejinting and concurrently appointed him as Chief Clerk to the Chancellor. Throughout Cao Pi's reign, he constantly plotted to usurp power and declare himself emperor, a plan Sima Yi fully understood, so he offered many suggestions. After Cao Pi successfully ascended the throne as emperor, Sima Yi was appointed as Minister of the Secretariat, soon transferred to Supervisor of the Army, then promoted to Censor-in-Chief, Palace Attendant, and Right Vice Director of the Secretariat, and was enfeoffed as Marquis of Anguo Village.
Before his death, Cao Pi entrusted the crown prince Cao Rui to Sima Yi and the three great generals Cao Zhen, Cao Xiu, and Chen Qun. After Cao Rui ascended the throne, he was historically known as Emperor Ming of Wei, and Sima Yi was enfeoffed as the Marquis of Wuyang and appointed as General of Agile Cavalry.
Since the loss of Jieting, Zhuge Liang led multiple northern campaigns against Wei, and Sima Yi personally commanded large forces to counter him. In 229 AD, Zhuge Liang attacked Wei's Wudu and Yinping commanderies. Sima Yi, as Grand General, led troops to confront him, while Cao Zhen led the main force from Chang'an as reinforcements, Left General Zhang He took a detachment through Xie Valley, and Sima Yi crossed the Han River from Jingzhou to reach Xicheng (present-day northwestern Ankang, Shaanxi). The three armies converged at Hanzhong. Sima Yi advanced on Shu commanderies by both water and land, reaching Xinfeng, where the Shu army suffered a major defeat. Following days of heavy rain, Sima Yi judged the weather unfavorable for combat and, seeing the Shu forces had sustained heavy losses and expecting no further counterattack from Zhuge Liang, withdrew his troops back to court.
In 231 AD, after a period of rest and reorganization, Zhuge Liang once again launched a northern expedition, besieging the Qishan region. Sima Yi immediately led his troops to garrison Chang'an, while other generals prepared to resist the Shu army. The battle was fiercely fought, with the Shu army harvesting the wheat from the occupied lands before the Wei army could, ensuring their troops did not suffer from food shortages and giving them enough time and energy to contend with the Wei forces. However, a strategic error by Zhuge Liang put the Shu army at a disadvantage, and soon after, Zhuge Liang died of illness.
In 234 AD, Zhuge Liang died of illness at Wuzhangyuan (present-day Wuzhangyuan, Qishan, Shaanxi), bringing an end to his seven-year Northern Expeditions; this prolonged campaign ultimately ended with a Wei victory, and Sima Yi was promoted to Grand Commandant for his achievements, though he originally planned to press on and attack Shu Han, Emperor Ming of Wei stopped him.
In 235 AD, the local warlord Gongsun Du, who controlled Liaodong, caught Sima Yi's attention due to his wavering attitude toward the Wei Kingdom—sometimes agreeing to surrender, other times rebelling, and eventually growing increasingly hostile. After Gongsun Du's grandson, Gongsun Yuan, became the Governor of Liaodong, he showed no respect for Wei whatsoever. By 237 AD, Gongsun Yuan openly declared independence from Wei, proclaimed himself King of Yan, and established his capital at Xiangping (present-day Liaoyang, Liaoning).
After receiving permission from Emperor Ming of Wei, Sima Yi led a grand force of forty thousand cavalry and infantry, departing from the capital and marching for several months to reach the border of Liaodong. Upon learning of the Wei army's approach, Gongsun Yuan panicked and hastily ordered his Grand General Bei Yan and others to deploy a defensive line stretching twenty li along the border.
However, Gongsun Yuan's defenses could not withstand the Wei army's attack. Sima Yi deliberately erected many banners in the south, feigning preparations for an assault, causing Gongsun Yuan's forces to concentrate their main strength on southern defenses. Meanwhile, the main Wei army had already crossed the Liao River under cover, bypassing the Liao army's camp and heading straight for Xiangping. This caught Gongsun Yuan completely off guard. In the ensuing panic, the troops stationed at the Liao River hastily rushed back to Xiangping, but they were no match for the well-prepared Wei forces, suffering defeat in three consecutive engagements. In the end, Sima Yi successfully secured victory in the Liaodong campaign, capturing Xiangping in one decisive strike.
But just at this moment, an urgent letter from Emperor Ming of Wei arrived from the capital, ordering Sima Yi to return to Luoyang immediately upon receiving it. Sima Yi guessed that something very important must have happened at court, so he traveled day and night to rush back. After arriving in Luoyang, Sima Yi learned that Emperor Ming of Wei was already gravely ill and lying on his deathbed, barely breathing. Sima Yi hurried into the palace, where Emperor Ming, with only his last breath left, called Sima Yi and the imperial relative Cao Shuang to his bedside and ordered them to properly assist the crown prince, Cao Fang. After saying this, Emperor Ming passed away.
Crown Prince Cao Fang ascended the throne as Emperor Shao of Wei, with Sima Yi serving as Grand Commandant and Cao Shuang as Grand General; their power was evenly matched, each commanding their own military forces. Initially, Cao Shuang showed great respect for Sima Yi, consulting him on all matters, but over time, he grew weary of this and, encouraged by his close advisors, gradually developed ambitions to monopolize power. Cao Shuang devised schemes to trick the young emperor into demoting Sima Yi to the position of Grand Tutor, stripping him of all real authority and military command, thus neutralizing any threat he posed. Sima Yi, aware of Cao Shuang's intentions, outwardly remained composed, but inwardly he had already formulated his own plans.
General Cao Shuang, having taken command of all the military forces of the Wei Kingdom, could not help but become arrogant and overconfident. Ignoring Sima Yi's warnings, he forcibly led his troops to attack the Shu Han regime, but was defeated by the Shu general Wang Ping, nearly losing his entire army and barely managing to flee back to Luoyang in disgrace.
In 247 AD, Cao Shuang, following the advice of his trusted confidants, conspired with Cao Xi to imprison the Empress Dowager in Yongning Palace and forced the young Wei emperor Cao Fang to appoint his own relatives and loyalists to key positions, so that within a short time, all officials both inside and outside the court were Cao Shuang's men. Cao Shuang also repeatedly altered the laws, acted arrogantly and tyrannically everywhere, and regarded himself as supreme, inciting strong resentment among other ministers.
Sima Yi pretended not to want a direct confrontation with Cao Shuang, so he submitted a memorial to the young emperor, resigning from his position as Grand Tutor and returning home to recover from illness. Cao Shuang, still uneasy, sent someone to investigate Sima Yi's condition, and only after learning that Sima Yi was truly gravely ill did the stone weighing on his heart finally drop. From then on, Cao Shuang became even more reckless and unrestrained. In reality, while Sima Yi appeared to be recuperating, he was secretly gathering many influential ministers, preparing to find an opportunity to eliminate Cao Shuang.
In 249 AD, the young Emperor of Wei, accompanied by Cao Shuang, Cao Xi, and several ministers, went to pay respects at Emperor Ming of Wei's tomb. Upon hearing this news, Sima Yi, full of vigor, donned his armor and went to the palace to meet the Empress Dowager, requesting her to issue an order dismissing Cao Shuang and his associates from their positions. When Cao Shuang and his men received the Empress Dowager's decree outside the city, they were terrified and at a loss, pleading desperately with the young emperor. However, the young emperor dared not defy the Empress Dowager's command and, upon returning to court, ordered the removal of Cao Shuang and his brother from their official posts and military authority. Sima Yi also eliminated all of Cao Shuang's trusted allies who had been placed within the court.
After Cao Shuang was deposed, he could only remain confined to his own residence, while Sima Yi stationed troops around the estate, sealing it off completely to prevent Cao Shuang from escaping. Later, Sima Yi sentenced Cao Shuang to death on charges of treason, and his younger brother along with several close confidants were also sentenced to death, with their entire clans exterminated to the third degree of kinship. From then on, the power of the Wei Kingdom fell into the hands of the Sima family.
After Sima Yi's death, his son Sima Shi inherited his position and continued to monopolize power. In 254 AD, Sima Shi learned that Li Feng, Zhang Ji, and others were plotting to depose him, so he unhesitatingly eliminated them; the young Wei emperor was also implicated, deposed by Sima Shi and demoted to the Prince of Qi. Subsequently, fourteen-year-old Cao Mao was installed as emperor, with all power firmly in Sima Shi's hands.