Prior to The Party Crasher and Guzma the Destroyer Moon caught Mareanie, Grubbin, and Grimer somewhere near Iki Town in order to have Pokémon to compete with in the Iki Town tournament. Mareanie was first seen alongside Grimer when Moon showed Gladion the team she was going to use to battle him in the tournament. After Grimer was defeated, Mareanie was sent out to fight Porygon. Taking advantage of her speed, Mareanie ran around the stage until Grimer's Poison Gas took effect, slowing Porygon down. Although Mareanie managed to dodge Psybeam, she was defeated when it used Zap Cannon, making Gladion the winner of the battle.
In Homecoming and the Brilliant Professional Golfer, Mareanie helped Moon alongside the rest of her Pokémon and Mallow's Steenee in their quest to find the Mirage Berry that was needed for Tapu Lele. By asking the local Pokémon of Lush Jungle, they made quick work of the task and found the Berry.
In Unleashing the Incredible Z-Move, when Tapu Lele asked the group to battle it so it could test their strength, Mareanie battled the guardian deity alongside Dollar, Kiawe's Marowak, and Kahili's Skarmory. Despite the combined effort of their attacks, Tapu Lele easily blew the group away. However, Dollar's Trainer refused to give up, and he subsequently evolved into a Torracat, allowing it to land a hit on Tapu Lele. While the attack didn't harm Tapu Lele, it acknowledged their determination and flew off.
In Flash and Cosmog's Secret, Mareanie battled the Totem Pokémon Lurantis and its underlings, which included Castform, Trumbeak, Comfey, and Kecleon. After a grueling fight that lasted for hours, Mareanie defeated the Totem Pokémon with Poison Jab.
In Battle in Vast Poni Canyon, Mareanie was sent out alongside the rest of Moon's team to fight a group of Aether Foundation Employees at Vast Poni Canyon. They were later switched out for Anabel's team.
In Darkness!! The Mysterious Claws of Necrozma!, Mareanie was used alongside the rest of Moon and Sun's teams to assist Lunala in the fight against Necrozma, but was stopped by Lusamine's Naganadel.
After Moon exited Ultra Space, Mareanie and the rest of Moon's team were stolen by Faba and stored at Aether Paradise in Secret Lab B. In Regeneration!! The Power of the Sun and the Moon!, after Moon escaped Faba, she headed to Secret Lab B and found Mareanie and the others safe. While trying to prepare medicine for Gladion, Moon was ambushed by Aether Foundation Employees and had her team hold them off while she finished her work. Mareanie battled an Alolan Raticate and defeated it.