Theo Magath ascended to general in the Marleyan military after his predecessor Calvi's death, backed by Willy Tybur, secret ruler of Marley. For years, Magath commanded the Eldian Warrior Unit, evaluating candidates and assigning inheritors for the Nine Titans—Annie Leonhart, Bertholdt Hoover, Marcel Galliard, Pieck Finger, and Reiner Braun—though he questioned entrusting the Paradis Island Operation to children despite their combat prowess.

Driven by duty to Marley, Magath consistently prioritized strategy over individual lives. His pragmatism manifested during the Battle of Fort Slava, where he sacrificed 800 Eldian soldiers to eliminate anti-Titan artillery while shielding Titan Shifters and Warrior candidates like Gabi Braun and Falco Grice. This choice reflected his assessment that Marley’s military power depended on Titans amid rivals' technological progress, recognizing Titan overreliance had stunted conventional development.

Despite sharing Marleyan prejudices against Eldians, Magath displayed atypical tolerance. He permitted Eldian subordinates such as Colt Grice and Zeke Yeager to voice tactical input without physical reprisal. His collaboration with Eldian leader Willy Tybur underscored this pragmatism; together, they orchestrated the Liberio festival to bait Paradis forces, acknowledging mutual complicity by declaring both Marleyans and Eldians "devils." Magath later directed Marley’s counterattack against Eren Yeager’s assault on Liberio.

After the declaration of war and the Rumbling’s onset, Magath’s outlook shifted. He forged an alliance with Paradis soldiers, including Keith Shadis, to halt global destruction. This cooperation involved freeing Azumabito engineers to repair an airship and sabotage Marley’s fleet targeting Paradis. During the mission, Magath voiced regret for exploiting Eldian children as Warriors, wishing they had normal lives. He ultimately sacrificed himself against Yaegerist forces, detonating explosives to cover the engineers’ escape and perishing alongside Shadis.