Munemori ascended to leadership of his clan following the death of Kiyomori in 1181, a year after the start of the Gempei War. Kiyomori's eldest son, the popular and talented Shigemori had predeceased his father (1179), leaving the much less capable Munemori to take his place. Initially, Munemori inherited a war in a state of limbo - poor weather had devastated the crops of 1180 and 1181, forcing both sides to cease hostilities. Prior to the enforced cease-fire, Minamoto (Kiso) Yoshinaka had led an army out of Shinano, marching into Etchu, Kaga, and Echizen. At the same time, an army under Minanoto Yukiie had been crushed in Owari while operations in the Kanto were largely inconclusive. The war seemed as if it could go either way, and easily to the Taira had a man of ability led them. Unfortunately for the Taira, this hardly proved the case.
When the rice stores had sufficiently recovered to allow offensive action, Munemori dispatched his son Koremori and brother Michimori to Echizen with the aim of rolling back Yoshinaka. The campaign got off to a decidedly bad start-evidently, feeding the mass of poorly trained levies the Taira was fielding presented a logistical problem they elected to forego. According to the Tale of the Heike, the Taira army became a ravenous swarm of locusts before they even reached Echizen, a cruel thing indeed to unleash on a peasantry hardly recovered from over a year of famine. Needless to say, the resulting Battle of Kurikara (June 1183) ended as a crushing Taira defeat. In one masterful stroke, Yoshinaka brought the war to a turning point and sent the fortunes of the Taira spiraling. With Yoshinaka's banners headed for the capital, Munemori asked the warrior-monks of Mt. Hiei for assistance. When the Enryakuji replied with a strong refusal, Munemori packed up the Child-Emperor Antoku, most of the royal family, and the Royal Regalia, and fled Kyoto. Yoshinaka arrived fast on his heels and installed the former Emperor Go-Shirakawa to the throne.
The Taira busied themselves preparing a defense of the western provinces into 1184 while Minamoto Yoritomo eliminated Yoshinaka (who had now become more trouble to the Minamoto then he was worth). It may be that Munemori hoped that this split would bode well for the Taira - he maintained a forward base at Ichi-no-tani in Settsu, just in case the opportunity for a counter-attack presented itself. In March 1184, however, the legendary Minamoto Yoshitsune captured Ichi-no-tani, and the war entered its final stages as the gate to the west was thrown open. Minamoto Noriyori was dispatched to march overland while Yoshitsune prepared for an assault on Munemori's Shikoku headquarters - Yashima (Sanuki province). A year after the fall of Ichi-no-tani, Minamoto Yoshitsune crossed over to Shikoku in a driving storm and attacked Yashima. The battle was not costly in terms of blood, but it resulted in another Taira retreat - the last possible. When Munemori and his by now disheartened clan reached their base at Hikoshima in the Shimonoseki Strait they had run out places to flee to. Beyond lay only the sea, and Yoshitsune was hard on their heels. In April he arrived, and on the morning of 25th the final battle of the Gempei War commenced. Outnumbered both in men and ships, the Taira situation looked bleak. At this critical juncture, Munemori made a compounding error. Among his generals there was a certain Taguchi Shigeyoshi, whom Taira Tomomori suspected of disloyalty. Tomomori asked that Taguchi be executed just to be on the safe side - a request Munemori refused.
The Battle of Dan-no-ura proved a bitter struggle. The Taira had nothing to lose and so fought with an élan that recalled their glory days. Determined to kill Yoshitsune, Tomomori fought bravely and well, but, just as he feared, Taguchi switched sides and revealed which ship the Emperor Antoku could be found on. Combined with a change in the tides, this event sealed the battle and many Taira, having fought all they could, threw themselves into the sea. Tomomori hacked his way back to the Emperor's ship and informed Antoku and his mother that all was lost, the two leaping into the ocean to drown. For his part, Munemori was a little lukewarm on the notion of suicide and so hesitated, despite the curses of his own mother (who accused him of actually being the son of an umbrella salesman - literally). One of his own men tried to make Munemori give a good accounting of himself by shoving him overboard. It turned out that Munemori was a better swimmer then a leader, and was later fished out of the water by the victorious Minamoto. He was later taken to Kyoto and executed in shame, one of the few Taira not to commit suicide at Dan-no-ura. Munemori became infamous for his ineptness and quite beneath contempt for not dying with his clan at Dan-no-ura on 25 April 1185.