Mamoru Seki came from a military family; his father was a captain in IJN and participated in Battle of Tsushima, while his uncle on his mother's side was Vice Admiral Sakonji Seizō.

Later on, Seki became a flight division leader (Buntaichō) in the dive bomber squadron of carrier Ryūjō and participated in Canton Operation during the Second Sino-Japanese War in 1938.

He was then transferred to Usa Air Group in Ōita Prefecture on Kyushu and served as an instructor when the Pacific War started.

The second strike wave, led by Lieutenant Sadamu Takahashi of Zuikaku, failed to locate the enemy carriers and therefore missed the opportunity to finish off Enterprise.

[4][3] He was one of many experienced carrier pilots lost in the battle; among them was his academy classmate Lieutenant Commander Shigeharu Murata, who led the first wave strike that crippled Hornet.