Mario and Pauline have known each other since Donkey Kong. In the game, the title character, Donkey Kong—later confirmed to be a younger Cranky Kong—kidnaps Pauline. Mario, similar to Peach and Daisy, saves Pauline. During this period of time, Pauline was merely known as 'Lady' and blonde. Mario, on the other hand, went by 'Jumpman' and 'Mr. Video Game' at the time. It would appear the pairing originally were lovers, but eventually broke up. They are still on good terms as shown with Super Mario Odyssey.
Years after the events of Donkey Kong, in Mario vs. Donkey Kong 2: March of the Minis, Mario has Pauline as a guest of honor. She cuts the ribbon to the new Super Mini Mario World theme park. However, after Pauline chooses a gift Mario gave her over Donkey Kong's, she is once again taken away. Mario saves Pauline, and she ends up accepting Donkey Kong's gift as well. In the manual, it is heavily implied they are now friends here.
In Super Mario Odyssey, Pauline is now the singing mayor of New Donk City. Here, she will ask Mario to help find band members for a tribute towards Mario. Once completed, Mario will be able to go through multiple 8-bit stages themed around his times as Jumpman. Pauline, on the other hand, will be singing 'Jump-Up, Super Star,' featuring lyrics in reference to Mario's history as a hero.
In terms of reasons the pairing is ship, it is a canon fact they have dated in the past. Some also see a nostalgic appeal, due to how Donkey Kong was both Mario and Pauline's first appearance in any form of media. However, when compared to other ships, the pairing is fairly underground in popularity.
As of now, on AO3, there are around five fics featuring the pair, making it the second most popular ship for Pauline, behind Peachauline.
When following the fan theory that Jumpman (Mario from the Donkey Kong arcade games) is Mario's father, Mario and Pauline can be seen as a familyship instead. Pauline can be seen as either Mario's mother using makeup to make herself look younger or Mario's older sister who, like Mario, was named after one of their parents. In the Mario vs. Donkey Kong series, Mario invited Pauline as a guest of honor, and some fans felt it would make more sense for Mario to invite his sister or mother than his ex-lover.
|SHIPS||het||Blippi • Luaisy • Luich • Maraisy • Mareach • MariPauline • Marivian • Powser|
|slash||Bowsario • Bowuigi • Genario • Luisley • Walgi • WariMari|
|femslash||Cacklebean • Daisylina • Peachalina • Peachauline • Peaisy|
|poly||Bowsareach • Bowuisley • Peachaisalina|
|CHARACTERS||male||Bowser • Luigi • Mario|
|female||Princess Daisy • Princess Peach • Rosalina|