Before the fall of Wall Maria, Mr. Arlert and his wife managed to create a hot air balloon. However the Military Police Brigade found it out and killed them as they were trying to escape with their device. One man in the family kept a secret book which contained forbidden knowledge of the world beyond the Walls, with descriptions of fiery lakes and snowfields of sand. Eventually, the man gave this book to his grandson Armin Arlert. When he was nine years old, Armin showed this book to his friend Eren Yeager, inspiring him to see the outside world.
When Shiganshina District is attacked by the Colossus Titan in the year 845, Armin Arlert and his grandfather escaped. When Eren and Mikasa run to their house in the carnage to see if their mother is safe, Armin warns Garrison soldier Hannes of their peril, saving them from being eaten by Titans.
One year after Shiganshina District is overrun by the Titans, the Military issues a culling disguised as a reclamation expedition. In this expedition, Armin's grandfather is killed. Armin would spend the next year with his friends Eren and Mikasa, working the fields to provide food for the people of the Walls.
On the day before graduation, Armin is nearly eaten when Trost District is attacked by the Colossus Titan and many Titans enter the city. He is saved by Eren, who is thought to have died in the process. However, it is later discovered that Eren survived due to his Titan powers. With this knowledge, Armin formulates a plan to save Trost with Eren's powers, and with the help of the trainees and the Garrison the plan succeeds.
After graduation, Armin joins the Survey Corps and partakes in the 57th expedition beyond the Walls, where he discovers that the Female Titan that invades the expedition is in fact his former trainee comrade Annie Leonhart. After bringing this information to Commander Erwin Smith, Annie is successfully captured in Stohess District.
During the Survey Corps operation to retake Shiganshina, Armin is placed in command of a team to search for the enemy, with Erwin Smith noting he is one of their greatest weapons. Later, he is given command of the Special Operations Squad in Levi's absence, though later relinquishes this to Jean Kirstein. When Armin is left on the brink of death at the battle's conclusion, he is given the Titan injection and consumes Bertolt Hoover, inheriting the power of the Colossus Titan into the Arlert family.