Mantis was raised by the living planet Ego from when she was young.
The Guardians of the Galaxy members Gamora, Drax the Destroyer, Rocket Raccoon, and Groot encounter Mantis where she takes them through the tunnels beneath the Spartaxian Royal Palace in order to save Star-Lord. They go through various obstacles where Mantis tells them to believe. After the group frees Star-Lord, they end up in the throne room where the Cosmic Seed energy is even after a fight with the robots there and a brief fight with Emperor J'son. When Mantis claims that J'son is a tyrant, she tries to attack him only to be subdued by the Guardians of the Galaxy. While being remanded to the dungeon, Mantis still claims that J'son is a tyrant and that they will eventually believe that fact.
Later on Mantis had escaped from the Spartaxian Dungeon and has joined her cult, the Universal Believers's fellow members in a criminal only auction house on Knowhere. While accessing the vault in Knowhere's Sinus Cavity to look for the Pocket Dimension Vial, Rocket Raccoon and Drax the Destroyer run into Mantis who has gained Ebony Maw of the Black Order as the latest member of the Universal Believers. They defeat Rocket Raccoon and Drax the Destroyer and then have the Blood Brothers dispose of them on a nearby planet. Upon the two of them being rescued by Star-Lord, Gamora, and Groot, Rocket Raccoon and Drax the Destroyer confront Mantis and the rest of the Universal Believers on their ship that was orbiting another planet. They used a device which converts mental energy into physical energy to aid them in combat. With the help of Rocket Raccoon's lies, the Guardians of the Galaxy cause doubt in the cult's belief and thus removes their powers allowing them to defeat Mantis, Ebony Maw, and the rest of the Universal Believers fanatics.