Little is known of Theodric's life and reign, although Urien, the king of Rheged, is said to have subjected Theodric and his sons to a three day siege on the island of Lindisfarne.

According to medieval Welsh poetry (such as the poem Gweith Argoed Llwyfain [The Battle of Argoed Llwyfain or Battle of Leeming Lane] from the Book of Taliesin), Theodric is killed in battle by Urien's son, Owain mab Urien, after Theodric demands hostages and Owain refuses to give in.

Theodric is succeeded by his brother Frithuwald. (The earliest authorities differ widely on the order and the regnal years of the kings between the death of Ida in 592/593 and the beginning of Æthelfrith's rule.)