When Mamoru was six, he was seriously injured in an automobile accident that left him with amnesia (he remembers very little of his childhood, and who he really is). Neither his mother nor his father survived the accident.  Although he was once employed as a model, Mamoru is not shown working at a full-time job. Presumably, he was left an inheritance by his parents that pays for his sports car, motorcycle, and decadent apartment in which he lives alone. Mamoru dated Rei a few times before he learned of his past life as Princess Serenity's true love during the Silver Millennium. Mamoru's heart now belongs to Usagi, who affectionately refers to him as "Mamo-chan". Mamoru prefers to only rely on himself, and often hides his feelings from the Sailor Senshi and his close friend, Motoki. In 30th century Crystal Tokyo, Mamoru becomes king of the Earth and rules the planet with his wife, Neo-Queen Serenity.

Mamoru is the reincarnation of the Prince of the Earth, and transforms into the dashing hero, Tuxedo Mask (known as Tuxedo Kamen in Japan). However, Mamoru did not realize that he had transformed into Tuxedo Mask whenever the Sailor Senshi were in danger until the middle of the first season. He attends collage at the Azabu Institute of Technology during the day, but can appear as Tuxedo Mask to help the Scouts at any time of the day or night. Shortly after his Tuxedo Mask identity is revealed to the Sailor Scouts, Mamoru is captured by the Negaverse and placed under Queen Beryl's Mind control. Mamoru's free will is returned only after Sailor Moon's love reaches deep within his heart. Additionally, when Mamoru forgets that he is Tuxedo Mask at the beginning of the second season, his soul manifest itself as  the psychic projection, the Moonlight Knight, to protect the Sailor Senshi.