Both in his service to his grandnephews and in his own reign as emperor, he is generally viewed as a weak-willed figure who showed enough wisdom to continue to survive and extend Jin rule, but whose effectiveness was also compromised by his over-dedication to philosophical discussions of Taoism and other related philosophies.
Sima Yu was born in 320, as the youngest son of Emperor Yuan, by his favorite concubine Consort Zheng Achun (鄭阿春).
As Emperor Yuan's wife Yu Mengmu (虞孟母) had died years earlier (in 312), and the mother of his oldest son Sima Shao the Crown Prince (later Emperor Ming), Lady Xun, had been forced to leave the palace due to Princess Yu's jealousy while she was still alive, Consort Zheng was effectively the mistress of the palace, although she never carried the title of empress.
In 326, Consort Zheng died, and because, as Prince of Langya, Sima Yu was not permitted by law to mourn his mother, he, at age six, requested another title by which he could.
Emperor Mu's mother Empress Dowager Chu became regent, but she largely followed the advice of prime minister He Chong (何充).
Sima Yu thus invited the official Yin Hao, who was considered able, to join him and Cai in making important decisions, to counter Huan's influence.
Around the new year 352, Huan, impatient after his requests to advance north in light of Later Zhao's collapse were being rebuffed by Sima Yu and Yin, who were concerned about his expanding power, and mobilized his troops and gestured as if he were about to attack the capital.
Yin was shocked, and initially considered either resigning or send the imperial banner of peace (Zouyu Fan, 騶虞幡) to order Huan to stop.
In 363, when Emperor Ai's mother Princess Dowager Zhou died, Sima Yu served as regent for the three-month mourning period.
In 369, after Huan Wen launched a major attack against Former Yan but suffered a devastating defeat at the hands of Murong Chui, he considered another way to showcase his power.
In winter 371, Huan forced Empress Dowager Chu to issue an edict deposing Emperor Fei and replacing him with Sima Yu, who took the throne with trepidation but felt he had no choice.