Mars is a planet that first appears in the animated short. In it, Pen (later changed to Finn) gets frozen by one of Ice King's frozen lightning bolts, and his mind gets transported back in time and to Mars by Abe Lincoln, who gives him the courage to believe in himself. Earth appears normal in the background, which is explained by the fact that this scene takes place "back in time."
The planet also makes a few appearances in the series Adventure Time. In "Too Young," Peppermint Butler tells the Earl of Lemongrab that "food comes from Mars." In "Sons of Mars," Finn and Jake encounter Martians, who are the inhabitants of the planet. The episode reveals a deity named Grob Gob Glob Grod and introduces Abe Lincoln, the King of Mars. In "Astral Plane (episode)," the planet avoids colliding with a comet after Grob Gob Glob Grod manages to divert its path towards Earth.
Only a small portion of Mars is shown in "Sons of Mars." The portion that is shown is an amphitheater which is used as a courthouse. The courthouse is surrounded by a force field which can be opened and closed. It is currently inhabited by Martians and was formerly ruled by Abe Lincoln.
After his assassination, Lincoln makes a trade with Death to become immortal, and he eventually becomes the king of Mars. More than 200 years before the start of the series, Magic Man creates havoc in Mars by doing multiple horrible acts like turning water into hair, bringing shadows to life, and conjoining the limbs of all Martian citizens. Magic Man is banished to Earth.
In "Son of Mars," Grob Gob Glob Grod attempts to capture Magic Man and bring him back to Mars. However, Magic Man turns Jake into a copy of himself to fool the Martians. Jake (currently transformed into Magic Man's image) is convicted and executed. After Lincoln finds out what really happened, he makes a deal with Death in which Jake will be brought back to life in exchange for Lincoln's immortality.
- There are Mars symbols on many of the structures. The symbol is a circle with an arrow emerging from it, pointing at an angle to the upper right, interpreted as the shield and spear from the ancient Roman war god Mars.
- In "The Lich," it is revealed that Mars is the third planet from the sun because the Sun's expansion resulted in the destruction of Mercury.
- It also looks similar to Cowboy Bebop's Mars, where humans also live in domes because the rest of Mars cannot be terraformed.