Theodor is the son of alderman Alois Xaver Ab Yberg and his wife Maria Anna von Reding.
In a dispute between the outer districts of Schwyz who demanded equal rights under the Constitution and the old lands, he fervently vindicated the supremacy of the latter.
When the council decided to occupy Küssnacht in the then existing district Schwyz outer lands in August 1833, Ab Yberg was the leading commander of the troops.
As a result of the attack on Küssnacht, the canton Schwyz was occupied by the confederate Swiss army.
With the fall of the Sonderbund, a special alliance between seven Swiss cantons, he disappeared from the political scene.