At some point in the distant past, Princess Serenity and Prince Endymion fell in love. But this caused a war between the Earth and the Moon causing the destruction of the Moon Kingdom. They are reborn in the present as Usagi Tsukino and Mamoru Chiba. They don't get along at first but soon fall in love again.
Mamoru took on the identity of Tuxedo Mask looking for the Silver Crystal for keys to his past since he lost his memories in a car crash. Usagi was awakened by Luna as Sailor Moon to fight the Dark Kingdom. As time went on they learned that Usagi was the princess. He is captured by Queen Beryl who ruled the Dark Kingdom and turned against Sailor Moon with a spell but Sailor Moon saves him and returns him to normal. They are 'killed' in their last battle with the Dark Kingdom but return to life with no memories of their adventures.
Usagi's memories are returned and is upset that Mamoru does not remember her. Part of Mamoru did split off to become the Moonlight Knight to aid Sailor Moon. In their last battle with the Makaiju, the Moonlight Knight is defeated and returns to Mamoru causing his memories to come back.
At the beginning of the Black Moon Arc, Mamoru starts having nightmares and visions that if he stays with Usagi she would die. His visions are usually correct, so he was petrified of losing Usagi. Because of this breaks up with her, but stills aids her in battle as Tuxedo Mask. They both become depressed and Usagi even suspected that Chibiusa was the cause of the breakup. They eventually do get back together. It is later revealed that it was his future self that sent the dream to test their bonds.
It is among the most popular shippings. The relationship between Usagi and Mamoru was a major plot point for the series causing there to be many fans. Other fans of the show do not ship them as they feel that the present-day Usagi and Mamoru's feelings for each other are merely a result of their past lives' love and not their current lives. However, this is not the case. They fell in love with one another because their personalities were the same as in their past lives. Furthermore, Usagi and Mamoru developed an unspoken attraction to one another in the beginning of Sailor Moon. They both demonstrated many symptoms of a crush on one another and eventually realized they meant a lot to one another before they knew about their past lives.
Given the great popularity of the franchise in yuri shipping and the status of this ship as the center of biggest part of het shipping in it, this ship and Mamoru in particular are notorious as targets of several het vs slash ship wars. Although this is mostly common in modern Western fendom, the earliest nods to it were made by the first director of the show during the premiere broadcast of the original anime, when he joked that he wanted to kill Mamoru, or at least reduce his screen time with Usagi x Rei shipping.
The MamoUsa ship is commonly pitted against the SeiUsa ship.